[HACK] RSS: Avoid initializing MHUv2 comms

RD-Fremont uses MHUv3 instead of MHUv2. SecureFW tries to run MHUv2
comms irrespective of the platform type. To avoid a hard fault and allow
SecureFW to continue to receive interrupts, the initialize_mhu function
is skipped in tfm_multi_core_hal_init function.

Signed-off-by: Joel Goddard <joel.goddard@arm.com>
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: I3b4d118f5095a9a024b007408b88e527e3cd786e