ARM: sunxi: select MFD_SUN6I_PRCM when sun6i arch support is enabled

Select the MFD_SUN6I_PRCM option when sun6i arch is enabled in order to get
the PRCM (Power/Reset/Clock Management) related drivers compiled.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>