clk: renesas: r8a7794: Add new CPG/MSSR driver

Add a new R-Car E2 Clock Pulse Generator / Module Standby and Software
Reset driver, using the CPG/MSSR driver core.  This will enable support
for module resets, which are not supported by the existing driver.

The old driver can still be used through a Kconfig option, to preserve
backward compatibility with old DTBs.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>