powerpc/mm/hash64: Don't test for machine type to detect HEA special case

Instead, check for FW_FEATURE_SPLPAR. This should be roughtly equivalent
as all pseries machiens that can have an HEA also support SPLPAR and
no other machine type does.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>