feat(ras): add support for OEM error injection support

Introduce firmware error injection support. In Linux there will be sysfs
entries under einj framework, that can be controlled for initiating
various RAS Error injection operations. Error injection in Firmware, is enabled
by selecting firmware-first option in sysfs entry. This allows kernel to write
into a designated area in EINJ buffer from which TF-A RAS module will retrieve
the error type and component for injection.

Signed-off-by: Glen Yeldho <glen.yeldho@arm.com>
Change-Id: I996de084a1bd0080f1f090beb41f65542f07fcab