MdePkg: Add PCD for UART default receive FIFO depth

MdePkg: Add PCD for UART default receive FIFO depth

PcdUartDefaultReceiveFifoDepth is added to indicate UART default
receive FIFO depth.
Type of UINT16 is enough for FIFO depth.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Heyi Guo <heyi.guo@linaro.org>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>