usb: gadget: r8a66597-udc: Make BUSWAIT configurable through platform data

usb: gadget: r8a66597-udc: Make BUSWAIT configurable through platform data

BUSWAIT is a 4-bit-wide value that controls the number of access waits
from the CPU to on-chip USB module. b'0000 inserts 0 wait (2 access
cycles) and b'1111 inserts 15 waits (17 access cycles, hardware
initial value), respectively.

BUSWAIT value depends on peripheral clock frequency supplied to on-chip
of each CPU, hence should be configurable through platform data.

Note that this patch assumes that b'0000 (0 wait, 2 access cycles) is
rerely used and considered as invalid. If valid 'buswait' data is not
provided by platform, initial b'1111 (15 waits, 17 access cycles) will
be applied as a safe default.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>