Introduce a driver to provide calls into secure monitor mode.

In the Amlogic SoCs these calls are used for multiple reasons: access to
NVMEM, set USB boot, enable JTAG, etc...
Acked-by: NMark Rutland <mark.rutland@arm.com>
Signed-off-by: NCarlo Caione <carlo@endlessm.com>
[khilman: add in SZ_4K cleanup]
Signed-off-by: NKevin Hilman <khilman@baylibre.com>