BaseTools: Check the max size for string PCD.
BaseTools: Check the max size for string PCD.

According to PCD_DATABASE_INIT in
edk2\MdeModulePkg\Include\Guid\PcdDataBaseSignatureGuid.h,
the max size for string PCD should not exceed USHRT_MAX 65535(0xffff).

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: ZhiqiangX Zhao <d5cb37baecf82d4076c20e2d5029706f03fe31a0@intel.com>
Cc: Liming Gao <6480311aeeb4b006862f6d13ebabddc03f51e507@intel.com>
Cc: Yonghong Zhu <78d981fe33a016071af5222c56d5cf77f30fb0b2@intel.com>
Cc: Bob Feng <f53a8c5610dbb66123cb988c78ae464e9da7199c@intel.com>
Reviewed-by: Bob Feng <f53a8c5610dbb66123cb988c78ae464e9da7199c@intel.com>

MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2