irqchip/mmp: Declare init functions in common header file

The functions icu_init_irq and mmp2_init_icu are exported
from this code, so declare them in the header file to avoid
the following sparse warnings:

drivers/irqchip/irq-mmp.c:248:13: warning: symbol 'icu_init_irq' was not declared. Should it be static?
drivers/irqchip/irq-mmp.c:271:13: warning: symbol 'mmp2_init_icu' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
[maz: fixup commit message]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220724222152.551850-1-ben-linux@fluff.org