Update inference runner IFM and OFM help message

The inference runner's help message has been updated to make it clear
what the expectations are for the IFM and OFM files. This also makes it
consistent with the delegate runner's help message.

Change-Id: I880d257bd77c3f31111353f4896899b81445badb
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>