efi: introduce efi_arch_flush_dcache_area

efi: introduce efi_arch_flush_dcache_area

Objects loaded by FileHandle->Read need to be flushed from dcache,
otherwise copy_from_paddr will read stale data when copying the kernel,
causing a failure to boot.

Introduce efi_arch_flush_dcache_area and call it from read_file.

This commit introduces no functional changes on x86.

Reported-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>