tc: Rework MMC file variable

Rework the variable which specifies the name of the model parameter for
the MMC file. Instead of passing a variable which specifies the whole
name, instead specify the model parameter path to the RoS (which
contains the MMC). This allows for making use of this RoS variable in
other parts of the common script.

Note that if the same IP block is instantiated in the different RoSs,
its parameters will have the same format and the only change will be the
RoS path (specified in the first part of the model parameter).

Signed-off-by: Jackson Cooper-Driver <jackson.cooper-driver@arm.com>