SCMI: Rename BUILD_HAS_FAST_CHANNELS with _SCMI_PERF_

The FastChannels functionality was introduced a while back to support
SCMI FastChannels as per SCMI spec v2.0. At that time only the
Performance protocol could advertise such support.
Now with the recent version 3.1[1] of the SCMI spec,also the Power
Capping protocol can support FastChannels.

It is now time to reflect which protocol the current FastChannel
support refers to, by adding _SCMI_PERF_ to its definition.
So:
- SCP_ENABLE_FAST_CHANNELS becomes SCP_ENABLE_SCMI_PERF_FAST_CHANNELS
- BUILD_HAS_FAST_CHANNELS becomes BUILD_HAS_SCMI_PERF_FAST_CHANNELS

This will allow precise inclusion of protocol-specific support for
SCMI FastChannels.

As a reminder, the CMake build option will become:
    make -f Makefile.cmake \
    MODE=<mode> \
    TOOLCHAIN=<toolchain> \
    PRODUCT=<product> \
    EXTRA_CONFIG_ARGS+="-DSCP_ENABLE_SCMI_PERF_FAST_CHANNELS=yes"

[1]https://developer.arm.com/documentation/den0056/d/?lang=en



Change-Id: I0ec4ffbc1ad5d797b9543aa979a98cceb267a798
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>