pinctrl: pinctrl-single: Avoid divisions in context save/restore

The divisions (and multiplications) can be avoided by changing the loops
to use increments of mux_bytes instead of 1.
While at it, remove the unneeded casts when assigning void pointers.

This saves +100 bytes of kernel size on arm32/arm64.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Tony Lindgren <tony@atomide.com>
Tested-by: Keerthy <j-keerthy@ti.com>
Reviewed-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>