ARM: 6467/1: amba: optional PrimeCell core voltage switch

On some contemporary sub-micron SoCs, peripherals on the chip have
power domain switches, i.e. the voltage to the core may be turned
off to conserve power. In the Ux500 we have this for out PrimeCell
derivates.

This patch makes it possible to specify an (optional) regulator to
handle the voltage domain switch on AMBA PrimeCells, modeled very
similar to how block clocks are handled.

Additional amba_vcore_[enable|disable] calls are supplied to make
it possible introduce optional powering off of the core voltage.
Using this will require code to spool/unspool any core HW state.

Cc: Rabin Vincent <rabin.vincent@stericsson.com>
Cc: Bengt Jonsson <bengt.g.jonsson@stericsson.com>
Cc: Jonas Aaberg <jonas.aberg@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>