gpio/mxc: remove redundant shadow variables initialization

With commit 3e11f7b8

 (gpio/generic: initialize basic_mmio_gpio shadow
variables properly) in place, the shadow variables initialization is
being done in generic driver bgpio_init call.

Remove the redundant shadow variables initialization from gpio-mxc
driver.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Tested-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>