pinctrl: ocelot: fix pinmuxing for pins after 31

The actual layout for OCELOT_GPIO_ALT[01] when there are more than 32 pins
is interleaved, i.e. OCELOT_GPIO_ALT0[0], OCELOT_GPIO_ALT1[0],
OCELOT_GPIO_ALT0[1], OCELOT_GPIO_ALT1[1]. Introduce a new REG_ALT macro to
facilitate the register offset calculation and use it where necessary.

Fixes: da801ab5

 pinctrl: ocelot: add MSCC Jaguar2 support
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>