efi_loader: restrict EFI_LOADER to armv7 and armv8 on ARM

fatload USB was reported to fail on the Sheevaplug. Debugging showed that
this was caused by an incorrect unaligned write to memory in
path_to_uefi().

UEFI on ARM requires that unaligned memory access is enabled.

* ARMv5 does not support unaligned access at all.
* ARMv6 supports unaligned access when we clear the A flag and set the
  U flag.
* On ARMv7 unaligned access is possible when clearing the aligned flag,
  which we do in function allow_unaligned() (arch/arm/cpu/armv7/sctlr.S).
  For none of the other cpus in arch/arm/cpu/ we have implemented a
  similar function.
* ARMv8 allows unaligned access.

Let EFI_LOADER on ARM depend on SYS_CPU=armv7 or SYS_CPU=armv8.

Once we have implemented allow_unaligned() for other ARM CPUs we can add
these to Kconfig.

Reported-by: Gray Remlin <gryrmln@gmail.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>