gpio: omap: Drop irq_base

The OMAP1 at one point was using static irqs but that time is gone,
OMAP1 uses sparse irqs like all other multiplatform targets so this
static allocation of descriptors should just go.

Cc: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Cc: Tony Lindgren <tony@atomide.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>