MAINTAINERS: Add an entry for common Renesas peripherals

MAINTAINERS: Add an entry for common Renesas peripherals

Renesas peripherals are common to SH4/RX based MCUs. Their
datasheets share common sections. It makes sense to maintain
them altogether.
Add the uncovered UART SCI peripheral.
The current names are misleading (see the 'sh_' prefix).
In another series we will remove these peripherals with
the 'renesas_' prefix. Out of the scope of this change in
MAINTAINERS.

Cc: Magnus Damm <magnus.damm@gmail.com>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>