regmap: mmio: add register clock support

Some mmio devices have a dedicated interface clock that needs
to be enabled to access their registers. This patch optionally
enables a clock before accessing registers in the regmap_bus
callbacks.

I added (devm_)regmap_init_mmio_clk variants of the init
functions that have an added clk_id string parameter. This
is passed to clk_get to request the clock from the clk
framework.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>