gpio: mvebu: kill bogus dependency on GPIO_GENERIC

The driver gpio-mvebu.c does not depend on gpio-generic.c at all.
Drop unneeded "select GPIO_GENERIC".

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>