pinctrl: sh-pfc: r8a7740: Remove obsolete multi-platform check

Since the removal of the r8a7740 legacy SoC code in commit
44d88c75 ("ARM: shmobile: Remove legacy SoC code for R-Mobile
A1"), r8a7740 is only supported in generic ARM multi-platform builds.
Hence CONFIG_ARCH_MULTIPLATFORM is always set, and the check can be
removed.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>