ARM: rockchip: force built-in regulator support for PM

The rockchips suspend/resume code requires regulators to work,
and gives a compile-time error if they are not available:

arch/arm/mach-rockchip/built-in.o: In function `rk3288_suspend_finish':
:(.text+0x146): undefined reference to `regulator_suspend_finish'
arch/arm/mach-rockchip/built-in.o: In function `rk3288_suspend_prepare':
:(.text+0x18e): undefined reference to `regulator_suspend_prepare'

To solve this, we now enable regulators whenever they are needed,
which is what we do on a lot of other platforms as well.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>