RISC-V: Create opensbi firmware domains

Incorporate with opensbi to create three firmware domains,
- Boot firmware domain, which built with opensbi library as
  M-mode access only region.
- Firmware domain which includes PEI and DXE regions, the
  PMP attribute is readable, wriable and executable.
- EFI Variable region which is readable and writable.

Cc: Sunil V L <sunilvl@ventanamicro.com>
Cc: Daniel Schaefer <daniel.schaefer@hpe.com>
Signed-off-by: Abner Chang <abner.chang@hpe.com>
Reviewed-by: Daniel Schaefer <daniel.schaefer@hpe.com>
Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>