pinctrl: Rename sh-pfc to renesas

The drivers/pinctrl/sh-pfc subdirectory was originally created to group
pin control drivers for various Renesas SuperH and SH-Mobile platforms.
However, the name "sh-pfc" no longer reflects its contents, as the
directory now contains pin control drivers for Renesas SuperH, ARM32,
and ARM64 SoCs.

Hence rename the subdirectory from drivers/pinctrl/sh-pfc to
drivers/pinctrl/renesas, and the related Kconfig symbol from
PINCTRL_SH_PFC to PINCTRL_RENESAS.

Rename the git branch in MAINTAINERS, too, for consistency.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200909131534.12897-3-geert+renesas@glider.be