ARM: mach-bcm: add ARM_ERRATA_775420

ARM_ERRATA_775420 needs to be enabled.

A data cache maintenance operation which aborts, followed by an ISB,
without any DSB in-between, might lead to deadlock.

Affects: bug present in Cortex-A9 r3p0 and earlier. Fixed in r4p0.

Cortex-A9 r3p0 is used in a multitude of Broadcom parts including
the bcm21664, bcm281xx, 5301x families of devices.

Signed-off-by: Scott Branden <sbranden@broadcom.com>
Tested-by: Markus Mayer <markus.mayer@linaro.org>
Reviewed-by: Markus Mayer <markus.mayer@linaro.org>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Matt Porter <mporter@linaro.org>