mmu: Fix mapping_pos calculation

In the MMU code mapping_pos is miscalculated when for example
SRAM_BASE_ADDRESS==0x40000000 and KERNEL_VM_SIZE==0xc0000000 getting a
mapping_pos of 0x0.

The problem is that we must cast the two values to uintptr_t before
casting the result to avoid the rollover to 0.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>