ARM: gemini: switch to sparse IRQs

There is no boardfiles or anything else using the fixed IRQs
anymore, switch the platform to use sparse IRQs and delete
the <mach/irqs.h> header.

Cc: Janos Laube <janos.dev@gmail.com>
Cc: Paulius Zaleckas <paulius.zaleckas@gmail.com>
Cc: Hans Ulli Kroll <ulli.kroll@googlemail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>