sgm775/run_model: align to changes in SCP firmware names

The SCP firmware binary names are now changed to scp_romfw.bin,
scp_ramfw.bin, mcp_romfw.bin and mcp_ramfw.bin. Align to these
changes in the run_model script.

Change-Id: If19f58e5516bcc6e09074b34471fa06fe0f495b2
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>