MdeModulePkg/CapsuleRuntimeDxe: Introduce PCD to control this feature.

MdeModulePkg/CapsuleRuntimeDxe: Introduce PCD to control this feature.

https://github.com/tianocore/tianocore.github.io/wiki/UEFI-Capsule-
on-Disk-Introducation

Introduce PcdCapsuleInRamSupport to turn on/off Capsule In Ram feature.
Platform could choose to drop CapsulePei/CapsuleX64 and not to support
Capsule In Ram.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Chao B Zhang <chao.b.zhang@intel.com>
Signed-off-by: Wei6 Xu <wei6.xu@intel.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>
Reviewed-by: Chao B Zhang <chao.b.zhang@intel.com>