ARM: ux500: make irqs.h local to platform

With the PRCMU out of the way, nothing outside of mach-ux500
uses mach/irqs.h any more, so we can make it a local header file,
including the two sub-headers.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>