ARM: ux500: move l2x0 init to .init_irq

The generic IRQ init function also enables the l2 cache
implicitly when the machine descriptor sets an .l2c_aux_mask.

Let's use that on ux500 and remove the ux500_l2x0_init()
along with the cpu_is_u8500_family checks.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>