common: splash_source: fix -Wint-to-pointer-cast warning

Fix -Wint-to-pointer-cast warning

common/splash_source.c: In function 'splash_load_raw':
common/splash_source.c:100:12: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
  100 |  bmp_hdr = (struct bmp_header *)bmp_load_addr;
      |            ^
common/splash_source.c: In function 'splash_sf_read_raw':
common/splash_source.c:39:47: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
   39 |  return spi_flash_read(sf, offset, read_size, (void *)bmp_load_addr);
      |                                               ^

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>