MdeModulePkg/SerialDxe: Set FIFO depth with PCD

MdeModulePkg/SerialDxe: Set FIFO depth with PCD

Set UART receive FIFO depth with PCD instead of fixed number "1".
The default value of PCD is also 1, so it makes no difference for
platforms which do not explicitly set this PCD.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Heyi Guo <heyi.guo@linaro.org>
Cc: Feng Tian <feng.tian@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>