ARM: mcpm: fix big endian issue in mcpm startup code

In big endian mode mcpm_entry_point is first function
that called on secondaries CPU. First it should switch
CPU into big endian code.

[ben.dooks@codethink.co.uk: merge fix patch from Victor into this]
Signed-off-by: Victor Kamensky <victor.kamensky@linaro.org>
Acked-by: Nicolas Pitre <nico@linaro.org>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>