regmap: mmio: Add function to attach a clock

regmap_init_mmio_clk allows to specify a clock that needs to be enabled
while accessing the registers.

However, that clock is retrieved through its clock ID, which means it will
lookup that clock based on the current device that registers the regmap,
and, in the DT case, will only look in that device OF node.

This might be problematic if the clock to enable is stored in another node.
Let's add a function that allows to attach a clock that has already been
retrieved to a regmap in order to fix this.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>