OvmfPkg/IndustryStandard: Introduce PageTable.h

We are going to use the page table structure in yet another place,
collect the types and macro that can be used from another module
rather than making yet another copy.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2490


Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
Message-Id: <20210412133003.146438-5-anthony.perard@citrix.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>