arm64: efi: Avoid dcache_clean_poc() altogether in efi_enter_kernel()

arm64: efi: Avoid dcache_clean_poc() altogether in efi_enter_kernel()

To allow efi_enter_kernel() to be shared with the EFI zboot decompressor
build, drop another reference to dcache_clean_poc() and replace it with
a single DC CVAC* instruction. To ensure that it covers the remainder of
efi_enter_kernel() as intended, reorganize the code a bit so it fits in
a 32-byte cacheline, and align it to 32 bytes. (Even though the
architecture defines 16 as the minimum D-cache line size, even the
chosen value of 32 is highly unlikely to ever be encountered on real
hardware, and this works with any line size >= 32)

* due to ARM64_WORKAROUND_CLEAN_CACHE, we actually use a DC CIVAC here

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>