mach-ux500: update SoC and board IRQ handling

This splits out a per-SoC IRQ range handling, so that the
DB8500 and DB5500 SoC:s can reuse aproximately the same IRQ
range with the largest span setting the roof. The same change
is done for the boards, mutatis mutandis, with a new file for
the U5500 board.

Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>