ARM: decompressor: run decompressor in place if loaded via UEFI

The decompressor can load from anywhere in memory, and the only reason
the EFI stub code relocates it is to ensure it appears within the first
128 MiB of memory, so that the uncompressed kernel ends up at the right
offset in memory.

We can short circuit this, and simply jump into the decompressor startup
code at the point where it knows where the base of memory lives. This
also means there is no need to disable the MMU and caches, create new
page tables and re-enable them.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Nicolas Pitre <nico@fluxnic.net>