x86: Add support for changing memory encryption attribute in early boot

Some KVM-specific custom MSRs share the guest physical address with the
hypervisor in early boot. When SEV is active, the shared physical address
must be mapped with memory encryption attribute cleared so that both
hypervisor and guest can access the data.

Add APIs to change the memory encryption attribute in early boot code.

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Tested-by: Borislav Petkov <bp@suse.de>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: kvm@vger.kernel.org
Cc: Borislav Petkov <bp@alien8.de>
Link: https://lkml.kernel.org/r/20171020143059.3291-15-brijesh.singh@amd.com