powerpc/vas: Move VAS API to book3s common platform

The pseries platform will share vas and nx code and interfaces
with the PowerNV platform, so create the
arch/powerpc/platforms/book3s/ directory and move VAS API code
there. Functionality is not changed.

Signed-off-by: Haren Myneni <haren@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/e05c8db17b9eabe3545b902d034238e4c6c08180.camel@linux.ibm.com