common: splash_source: Fix type casting errors

common: splash_source: Fix type casting errors

During compilation splash_source puts out below warning for type
conversion in splash_load_fit for bmp_load_addr and fit_header.
Change their type to uintptr_t to fix the warnings.

common/splash_source.c: In function ‘splash_load_fit’:
common/splash_source.c:366:22: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
  366 |         img_header = (struct legacy_img_hdr *)bmp_load_addr;
      |                      ^
common/splash_source.c:376:49: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
  376 |         res = splash_storage_read_raw(location, (u32)fit_header, fit_size);
      |                                                 ^
common/splash_source.c:401:25: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
  401 |                 memmove((void *)bmp_load_addr, internal_splash_data, internal_splash_size);

The above warnings are generated if CONFIG_FIT is enabled.

Signed-off-by: Nikhil M Jain <n-jain1@ti.com>