vc/amd/agesa/f15tn: Declare `value` as constant in `GnbRegisterWriteTNDump()`

vc/amd/agesa/f15tn: Declare `value` as constant in `GnbRegisterWriteTNDump()`

Do not discard the const qualifier in `GnbRegisterWriteTNDump()` to fix
the compiler warning below.

        CC         libagesa/vendorcode/amd/agesa/f15tn/Proc/GNB/Modules/GnbInitTN/GnbRegisterAccTN.o
    In file included from src/vendorcode/amd/agesa/f15tn/Proc/GNB/Modules/GnbInitTN/GnbRegisterAccTN.c:53:
    src/vendorcode/amd/agesa/f15tn/Proc/GNB/Modules/GnbInitTN/GnbRegisterAccTN.c: In function 'GnbRegisterWriteTN':
    src/vendorcode/amd/agesa/f15tn/Proc/GNB/Modules/GnbInitTN/GnbRegisterAccTN.c:836:57: error: passing argument 3 of 'GnbRegisterWriteTNDump' discards 'const' qualifier from pointer target type [-Werror=discarded-qualifiers]
      836 |     GnbRegisterWriteTNDump (RegisterSpaceType, Address, Value);
          |                                                         ^~~~~
    src/vendorcode/amd/agesa/f15tn/Proc/GNB/Common/Gnb.h:68:35: note: in definition of macro 'GNB_DEBUG_CODE'
       68 |     #define  GNB_DEBUG_CODE(Code) Code
          |                                   ^~~~
    src/vendorcode/amd/agesa/f15tn/Proc/GNB/Modules/GnbInitTN/GnbRegisterAccTN.c:86:33: note: expected 'VOID *' {aka 'void *'} but argument is of type 'const VOID *' {aka 'const void *'}
       86 |   IN       VOID                *Value
          |            ~~~~~~~~~~~~~~~~~~~~~^~~~~
        CC         libagesa/vendorcode/amd/agesa/f15tn/Proc/GNB/Modules/GnbInitTN/PcieComplexDataTN.o
        CC         libagesa/vendorcode/amd/agesa/f15tn/Proc/GNB/Modules/GnbInitTN/PcieConfigTN.o
        CC         libagesa/vendorcode/amd/agesa/f15tn/Proc/GNB/Modules/GnbInitTN/PcieEarlyInitTN.o
        CC         libagesa/vendorcode/amd/agesa/f15tn/Proc/GNB/Modules/GnbInitTN/PcieEnvInitTN.o
        CC         libagesa/vendorcode/amd/agesa/f15tn/Proc/GNB/Modules/GnbInitTN/PcieLibTN.o
    src/vendorcode/amd/agesa/f15tn/Proc/GNB/Modules/GnbInitTN/GnbRegisterAccTN.c: At top level:
    cc1: note: unrecognized command-line option '-Wno-pragma-pack' may have been intended to silence earlier diagnostics
    cc1: all warnings being treated as errors

Found-by: gcc (Debian 11.3.0-3) 11.3.0
Change-Id: I2039cf66030030458bd247a31adc0621b9d033e6
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/64989
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>