MdeModulePkg: Fix incorrect commit introduced by commit SHA-1:052c98

MdeModulePkg: Fix incorrect commit introduced by commit SHA-1:052c98

The default value of PcdExtFpdtBootRecordPadSize is 0x20000
But the following commit in master update it to 0 by mistake.
SHA-1: 052c98ce246a1ffb0b4c5185a644aa9f902650f7
Subject: MdeModulePkg: Add ResetSystemPei PEIM

This patch is to restore the value.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>