sunxi: Add SUN6I_PRCM kconfig entry

Add simple and meaningful kconfig option for prcm.c
instead of using MACH type on Makefile.

PRCM (Power/Reset/Clock Management) is considered as a
Multi-Functional Device, so used the same on Kconfig definition.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>