sh-pfc: r8a7791: fix typo in MLB_CLK

The R8A7791 manual sometimes calls the signal MLB_CLK and sometimes MLB_CK; the
latter can only be encountered in the PFC section and  is probably  just a typo
(this  signal is always called MLB_CLK in the R8A7790  manual). Fix occurences
of MLB_CK throughout the R8A7791 PFC driver.

Based on original patch by Andrey Gusakov <andrey.gusakov@cogentembedded.com>.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>