of: Use proper types for checking memory overflow

Commit a67a6ed1
(of: Check for phys_addr_t overflows in early_init_dt_add_memory_arch)
corrected early_init_dt_add_memory_arch to account for overflows
but did so in an unclean way using ULONG_MAX. There is no
guarantee that sizeof(unsigned long) == sizeof(phys_addr_t).
Check against phys_addr_t instead.

Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Grant Likely <grant.likely@linaro.org>