ARM: pxa: add devicetree code for irq handling

Properly register on-chip interrupt using the irqdomain logic. The
number of interrupts is taken from the devicetree node. That includes
the following changes:

- cpu_has_ipr() was converted from an inline function to a static bool
variable, so it can be set using the "marvell,intc-priority" property
inside the device node of the tree.

- IRQ_BASE was converted from a macro to a runtime variable so that it
can be initialized dynamically from the DT init code.

- irq_base() now uses pxa_irq_base and just adds an offset.

Hence, there are now no compile-time fixed values used in case of DT
initialization.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>