arm64: efi: fix remap bug

The code in remap_regions was incorrectly counting the number of
remapped regions. It was taking the difference of two pointers to
efi_memory_desc_t entries. But in some cases, depending on the
EFI memmap version, the actual size of the entries are different
from sizeof(efi_memory_desc_t). This patch changes the pointers
to void* which makes the code a little cleaner looking anyway. The
count is now calculated as the difference between the two void*
pointers divided by the actual entry size.

Signed-off-by: Mark Salter <msalter@redhat.com>