pinctrl: bcm-iproc: Use SPDX header

This convert the BCM IPROC driver to use the SPDX header
for indicating GPL v2.0 only licensing.

Cc: Pramod Kumar <pramodku@broadcom.com>
Cc: Ray Jui <rjui@broadcom.com>
Cc: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Scott Branden <scott.branden@broadcom.com>
Link: https://lore.kernel.org/r/20191002130217.4491-1-linus.walleij@linaro.org