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>
