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 <zhiqiangx.zhao@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Cc: Bob Feng <bob.c.feng@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>