mvebu: activate more workarounds for ARM erratas

The Armada XP uses a Marvell PJ4Bv7 Processor for which already one
workaround for an errata is activated.
The Armada 285 uses a Cortex A9 r4p1 for which the Linux kernel provides
a workaround for ERRATA_764369, activate this.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>