forked from Mirrors/opensbi

Add a generic RPMI mailbox client driver which provides a MPXY channel. Initially, this driver only supports RPMI clock service group but can be extended to support multiple RPMI service groups. Signed-off-by: Rahul Pathak <rpathak@ventanamicro.com> Co-developed-by: Anup Patel <apatel@ventanamicro.com> Signed-off-by: Anup Patel <apatel@ventanamicro.com>