ARM: spear: unify mach/generic.h and mach/irqs.h

These are indeed easy to combine, as there are no conflicting
definitions in generic.h, and irqs.h will be obsolete once
we enable SPARSE_IRQ.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>