UefiPayloadPkg: define some PCD as DynamicEX PCD

UefiPayloadPkg: define some PCD as DynamicEX PCD

Define some PCDs as DynamicEX PCD to be used as global variable.
Because PcdUartDefaultBaudRate is defined as DynamicEX, remove the code
to set it in platformlib. That code was actually redundant.

Reviewed-by: Guo Dong <guo.dong@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Cc: Maurice Ma <maurice.ma@intel.com>
Cc: Benjamin You <benjamin.you@intel.com>
Signed-off-by: Zhiguang Liu <zhiguang.liu@intel.com>