fdtdec: Cast prior_stage_fdt_address with uintptr_t

At present prior_stage_fdt_address is declared as phys_addr_t. On
a 32-bit platform where phys_addr_t can be 64-bit, assigning its
value to gd->fdt_blob which is a pointer, can cause warnings.

Cast it to uintptr_t before the assignment.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Simon Glass <sjg@chromium.org>