common/board_r: remove warning in initr_mem for 64-bit phys_size_t

Since on powerpc phys_size_t can be unsigned long long, this printout
line can result in a not nice compile warning.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Acked-by: Simon Glass <sjg@chromium.org>