TWL4030: use symbolic ISR/IMR register names during twl_init_irq()

twl_init_irq() uses a bunch of magic numbers as register indices; this
has already led to several errors, fixed earlier in this patch series.
Now use descriptive macros instead of magic numbers.  This patch should
not change kernel behavior.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>