refactor(cpufeat): rename ENABLE_SPE_FOR_LOWER_ELS to ENABLE_SPE_FOR_NS

At the moment we hardcode the SPE functionality to be available on the
non-secure side only, by setting MDCR_EL2.E2PB accordingly.

This should be reflected in the feature selection symbol, so rename that
to ENABLE_SPE_FOR_NS, to make it clearer that SPE is not supported in
the secure world.

Change-Id: I3f9b48eab1a45d6ccfcbb9c90a11eeb66867ad9a
Signed-off-by: Andre Przywara <andre.przywara@arm.com>