The BPMP firmware driver never worked on big-endian kernels, and
cannot easily be made portable. Add a dependency to make this clear
in case anyone ever wants to try a big-endian kernel on this hardware.

Link: https://lore.kernel.org/linux-arm-kernel/Y34FCQ3xTmcjqKRT@orome/Signed-off-by: Arnd Bergmann <arnd@arndb.de>