Platform/ARM/ArmShellCmdRunAxf: switch to by-VA cache maintenance

Currently, the 'runaxf' shell command that exists only on ARM's own
development platforms deals with the caches in an unsafe manner, as
it relies on set/way maintenance to ensure that the ELF image it has
put into memory, as well as the running image itself is visible in
memory after the MMU and caches are disabled.

So let's switch to by-VA maintenance for the currently running image,
as well as the ELF image, and use a helper in assembly to ensure that
we are not relying on the stack between the disabling of the MMU and
the invocation of the ELF image.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>