x86: mmu: fix type mismatch of memory address in assert

In one of the ASSERT() statement, the PHYS_RAM_ADDR (alias
of DT_REG_ADDR()) may be interpreted by the compiler as
long long int when it's large than 0x7FFFFFFF, but is
paired with %x, resulting in compiler warning. Fix this
by type casting it to uintptr_t and use %lx instead.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>