MAINTAINERS: Add entry for Renesas arm64 architecture

Initial Renesas arm64 architecture support will be for the R-Car H3,
r8a7795, SoC and its Salvator-X board.

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