MAINTAINERS: renesas: Add "renesas," file contents pattern

Add a keyword match pattern for the word "renesas," in files to the
ARM/RISC-V/RENESAS ARCHITECTURE section.  This make sure patches
changing drivers that match against "renesas,<foo>" (as used mostly for
Renesas on-SoC components) are CCed to the linux-renesas-soc mailing
list.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/c1be1e97c5457eade25b0eb5118196677cecfc08.1679039809.git.geert+renesas@glider.be