[ARM] 4171/1: i.MX/MX1 optimize interrupt source retrieval

The macro "get_irqnr_and_base" in "entry-macro.S" optimized
according to Lennert Buytenhek suggestion.

Comments from Pavel Pisa:

 Sascha has approved patch some days ago

Signed-off-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>