efi: arm: reduce minimum alignment of uncompressed kernel

Now that we reduced the minimum relative alignment between PHYS_OFFSET
and PAGE_OFFSET to 2 MiB, we can take this into account when allocating
memory for the decompressed kernel when booting via EFI. This minimizes
the amount of unusable memory we may end up with due to the base of DRAM
being occupied by firmware.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>