clk: shmobile: mstp: Document mandatory compatible fallback

The generic MSTP gate clocks compatible value is mandatory, as the
driver uses only this value for matching.  Document that this is a
fallback that must be present.

Also fix a typo (missing plural "s") in the compatible value.

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