sb/intel/common/rcba_pirq: Use correct size_t length modifier

sb/intel/common/rcba_pirq: Use correct size_t length modifier

Building an image for the Lenovo T60 with `x86_64-linux-gnu-gcc-11`
fails with the format warning below.

        CC         ramstage/southbridge/intel/common/rcba_pirq.o
    src/southbridge/intel/common/rcba_pirq.c: In function 'intel_acpi_gen_def_acpi_pirq':
    src/southbridge/intel/common/rcba_pirq.c:86:69: error: format '%ld' expects argument of type 'long int', but argument 5 has type 'size_t' {aka 'unsigned int'} [-Werror=format=]
       86 |                 printk(BIOS_SPEW, "ACPI_PIRQ_GEN: %s: pin=%d pirq=%ld\n",
          |                                                                   ~~^
          |                                                                     |
          |                                                                     long int
          |                                                                   %d
       87 |                        dev_path(dev), int_pin - PCI_INT_A,
       88 |                        pirq_idx(pin_irq_map[map_count].pic_pirq));
          |                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          |                        |
          |                        size_t {aka unsigned int}

The return value of `pirq_idx()` is of type `size_t`, so use the
appropriate length modifier `z`.

Change-Id: I7af24cee536b81e4825b77942bcac75afeb9f476
Found-by: gcc (Debian 11.2.0-13) 11.2.0
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/60194
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>