clk: renesas: cpg-mssr: Add R7S9210 support

Add support for the R7S9210 (RZ/A2) Clock Pulse Generator and Module
Standby.

The Module Standby HW in the RZ/A series is very close to R-Car HW, except
for how the registers are laid out.
The MSTP registers are only 8-bits wide, there are no status registers
(MSTPSR), and the register offsets are a little different. Since the RZ/A
hardware manuals refer to these registers as the Standby Control Registers,
we'll use that name to distinguish the RZ/A type from the R-Car type.

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Acked-by: Rob Herring <robh@kernel.org> # DT bits
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>