feat(sme): fall back to SVE if SME is not there

Due to their interrelationship in the architecture the SVE and SME
features in TF-A are mutually exclusive. This means that a single binary
can't be shared between systems with and without SME if the system
without SME does support SVE, SVE will not be initialised so lower ELs
will run into trouble trying to use it. This unusual behaviour for TF-A
which normally gracefully handles situations where features are enabled
but not supported on the current hardware.

Address this by calling the SVE enable and disable functions if SME is
not supported rather than immediately exiting, these perform their own
feature checks so if neither SVE nor SME is supported behaviour is
unchanged.

Signed-off-by: Mark Brown <broonie@kernel.org>
Change-Id: I2c606202fa6c040069f44e29d36b5abb48391874