ARM: 7611/1: VIC: fix bug in VIC irqdomain code

The VIC irqdomain code added in commit
07c9249f
"ARM: 7554/1: VIC: use irq_domain_add_simple()"

Had two bugs:

1) It didn't call irq_create_mapping() once on each
   valid irq source in the slowpath when registering
   the controller.

2) It passed a -1 as IRQ offset for the DT case, whereas
   0 should be passed as invalid IRQ instead.

Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>