efi: x86: move mixed mode stack PA variable out of 'efi_scratch'

As a first step to removing the awkward 'struct efi_scratch' definition
that conveniently combines the storage of the mixed mode stack pointer
with the MM pointer variable that records the task's MM pointer while it
is being replaced with the EFI MM one, move the mixed mode stack pointer
into a separate variable.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>