drm/sun4i: dsi: Handle bus clock via regmap_mmio_attach_clk

drm/sun4i: dsi: Handle bus clock via regmap_mmio_attach_clk

regmap has special API to enable the controller bus clock while
initializing register space, and current driver is using
devm_regmap_init_mmio_clk which require to specify bus
clk_id argument as "bus"

But, the usage of clocks are varies between different Allwinner
DSI controllers. Clocking in A33 would need bus and mod clocks
where as A64 would need only bus clock.

Since A64 support only single bus clock, it is optional to
specify the clock-names on the controller device tree node.
So using NULL on clk_id would get the attached clock.

To support clk_id as "bus" and "NULL" during clock enablement
between controllers, this patch add generic code to handle
the bus clock using regmap_mmio_attach_clk with associated
regmap APIs.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20191222132229.30276-5-jagan@amarulasolutions.com