RAS: Add support for node registration

RAS: Add support for node registration

Previous patches added frameworks for handling RAS errors. This patch
introduces features that the platform can use to enumerate and iterate
RAS nodes:

  - The REGISTER_RAS_NODES() can be used to expose an array of
    ras_node_info_t structures. Each ras_node_info_t describes a RAS
    node, along with handlers for probing the node for error, and if
    did record an error, another handler to handle it.

  - The macro for_each_ras_node() can be used to iterate over the
    registered RAS nodes, probe for, and handle any errors.

The common platform EA handler has been amended using error handling
primitives introduced by both this and previous patches.

Change-Id: I2e13f65a88357bc48cd97d608db6c541fad73853
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>