ARM: ux500: remove irq_base property from platform_data

AB8500 GPIO no longer handles its GPIO IRQs. Instead, the AB8500
core driver has taken back the responsibility. Prior to this
happening, the AB8500 GPIO driver provided a set of virtual IRQs
which were used as a pass-through. These virtual IRQs had a base
of MOP500_AB8500_VIR_GPIO_IRQ_BASE, which was passed though pdata.
We don't need to do this anymore, so we're pulling out the
property from the structure.

Cc: arm@kernel.org
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>