kernel: mmu: fix boot address translation macros

The Z_BOOT_VIRT_TO_PHYS() and Z_BOOT_PHYS_TO_VIRT() address
translation macros are flipped in their calculations.
The calculation is supposed to be:

  virt = phys + ((KERNEL_VM_BASE + KERNEL_VM_OFFSET) -
                 SRAM_BASE_ADDRESS)

So fix the them.

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