[MIPS] Make I8259A_IRQ_BASE customizable

Move I8259A_IRQ_BASE from asm/i8259.h to asm/mach-generic/irq.h and
make it really customizable.  And remove I8259_IRQ_BASE declared on
some platforms.  Currently only NEC_CMBVR4133 is using custom
I8259A_IRQ_BASE value.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>