pinctrl: bcm281xx: make Kconfig dependency more strict

This driver is only useful on BCM281xx, so let the driver depend on
ARCH_BCM_MOBILE but allow compile coverage testing.
The main benefit is that the driver isn't available to be selected for
machines that don't have the matching hardware.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Sherman Yin <syin@broadcom.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>