pinctrl: bcm: Default PINCTRL_BCM281XX to y for ARCH_BCM_MOBILE

This driver is mandatory for proper operation on the platforms covered
by the ARCH_BCM_MOBILE Kconfig symbol, make sure we do that driver on
for these platforms.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>