pinctrl: armada-37xx: Stop using struct gpio_chip.irq_base

The Armada 37xx driver always initializes the IRQ base to 0, hence the
subtraction is a no-op. Remove the subtraction and thereby the last user
of struct gpio_chip's .irq_base field.

Note that this was also actually a bug and only worked because of the
above assumption. If the IRQ base had been dynamically allocated, the
subtraction would've caused the wrong mask to be generated since the
struct irq_data.hwirq field is an index local to the IRQ domain. As a
result, it should now be safe to also allocate this chip's IRQ base
dynamically, unless there are consumers left that refer to the IRQs by
their global number.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>