ARM: 8227/1: sa1100: switch to MULTI_IRQ_HANDLER

Add sa1100_handle_irq implementating handle_irq for sa1100 platform.
It is more or less a translation of old assembly code from assembler to
plain C. Also install this irq handler from sa1100_init_irq().

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>