ARM: mach-shmobile: optimize get_irqnr_preamble/and_base macros

Optimize the SH-Mobile interrupt assembly macros to avoid
reloading the INTFLGA register address in case of multiple
pending interrupts. Kindly pointed out by Russell King.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>