RiscV/ProcessorPkg: Create read mtime CSR library instances

Create library instances of reading Machine mode timer.
- MacineModeTimerLib is used to read mtime CSR through
  platfrom library.
- EmulatedMacineModeTimerLib is used to read mtime CSR
  through shadow CSR.

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>