Remove dependency between SPM_MM and ENABLE_SPM build flags

There are two different implementations of Secure Partition
management in TF-A. One is based on the "Management Mode" (MM)
design, the other is based on the Secure Partition Client Interface
(SPCI) specification. Currently there is a dependency between their
build flags that shouldn't exist, making further development
harder than it should be. This patch removes that
dependency, making the two flags function independently.

Before: ENABLE_SPM=1 is required for using either implementation.
        By default, the SPCI-based implementation is enabled and
        this is overridden if SPM_MM=1.

After: ENABLE_SPM=1 enables the SPCI-based implementation.
       SPM_MM=1 enables the MM-based implementation.
       The two build flags are mutually exclusive.

Note that the name of the ENABLE_SPM flag remains a bit
ambiguous - this will be improved in a subsequent patch. For this
patch the intention was to leave the name as-is so that it is
easier to track the changes that were made.

Change-Id: I8e64ee545d811c7000f27e8dc8ebb977d670608a
Signed-off-by: Paul Beesley <paul.beesley@arm.com>