staging: fsl-mc: irq: shorten name of interrupt name

For /proc/interrupts readability, platform bus MSIs are named
"ITS-pMSI" in the GIC ITS implementation for that bus.  Follow
a similar naming convention and call fsl-mc bus MSIs
"ITS-fMSI".

Signed-off-by: Stuart Yoder <stuart.yoder@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>