irqchip/mmp: Only touch the PJ4 IRQ & FIQ bits on enable/disable

Resetting bit 4 disables the interrupt delivery to the "secure
processor" core. This breaks the keyboard on a OLPC XO 1.75 laptop,
where the firmware running on the "secure processor" bit-bangs the
PS/2 protocol over the GPIO lines.

It is not clear what the rest of the bits are and Marvell was unhelpful
when asked for documentation. Aside from the SP bit, there are probably
priority bits.

Leaving the unknown bits as the firmware set them up seems to be a wiser
course of action compared to just turning them off.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Acked-by: Pavel Machek <pavel@ucw.cz>
[maz: fixed-up subject and commit message]
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>