MdeModulePkg Variable: Not get NV PCD in VariableWriteServiceInitialize

MdeModulePkg Variable: Not get NV PCD in VariableWriteServiceInitialize

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1323
Merge EmuVariable and Real variable driver.

Add macro NV_STORAGE_VARIABLE_BASE.
Not get NV PCD in VariableWriteServiceInitialize, but in
FtwNotificationEvent/SmmFtwNotificationEvent, then
VariableWriteServiceInitialize could be not aware the NV
storage is real or emulated.

This patch prepares for adding emulated variable NV mode
support in VariableRuntimeDxe.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>
Tested-by: Julien Grall <julien.grall@arm.com>
Acked-by: Julien Grall <julien.grall@arm.com>