clk: renesas: cpg-mssr: Add early clock support

Add support for SoCs that need to register core and module clocks early in
order to use OF drivers that exclusively use macros such as
TIMER_OF_DECLARE.

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>