pinctrl: sh-pfc: r8a73a4: Remove obsolete multi-platform check

As of commit 9d07d414 ("ARM: shmobile: r8a73a4: ape6evm: Remove
legacy platform"), r8a73a4 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: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>