ARM: pxa: fix build issue while CONFIG_OF is disabled

arch/arm/mach-pxa/built-in.o: In function `pxa3xx_dt_init_irq':
saar.c:(.init.text+0x604): undefined reference to `pxa_dt_irq_init'

Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>