Enable PMU tests on SGI-575

The model has been updated to a newer version which fixed the
issues causing the PMU tests to fail, so they are being re-enabled
on this platform.

Signed-off-by: Petre-Ionut Tudor <petre-ionut.tudor@arm.com>
Change-Id: Ie2c7a610432a13edb05630d319cd0e8d7148b834