clk: renesas: Introduce temporary compound clock for SCP compatibility

clk: renesas: Introduce temporary compound clock for SCP compatibility

The current state of SCP on Renesas R-Car Gen5 is not yet final and
is still missing full clock control, the clock control is exposed as
separate enable/disable and rate controls.

Temporarily introduce custom local compound clock, which are used as
an adaptation layer between U-Boot clock tree and current state of
SCP, and which bind two SCP clock into a single compound clock, which
provides both enable/disable and rate controls.

This is mainly meant to be used by SD/eMMC controller, to allow the
driver to both turn its clock on and off, and also obtain the current
clock rate. This is going to be removed once the SCP clock protocol
solidifies.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>