GPL-2. is not a valid SPDX identifier. Make it GPL-2.0

Fixes: 490e687e ("pinctrl: sh-pfc: Initial R-Car M3-N support")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Jacopo Mondi <jacopo+renesas@jmondi.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>