powerpc/pseries/svm: Add a powerpc version of cc_platform_has()

Introduce a powerpc version of the cc_platform_has() function. This will
be used to replace the powerpc mem_encrypt_active() implementation, so
the implementation will initially only support the CC_ATTR_MEM_ENCRYPT
attribute.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lkml.kernel.org/r/20210928191009.32551-5-bp@alien8.de