PM / Domains: R-Mobile SYSC: Document SH-Mobile AG5 (sh73a0) binding

SH-Mobile AG5 (sh73a0) can be handled by the existing bindings.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>