ARM: Use -mcpu=cortex-a9 when targeting U8500 CPUs

Enable compiler optimizations specific to the Cortex-A9
processor when targeting U8500 CPUs. This is necessary
take advantage of the UDIV/SDIV instructions supported by
these processors. To accomplish this, we need to remove the
-march=armv7-a ISA restriction from the compiler options
because 'cortex-a9' is a superset of 'armv7-a'.

Change-Id: I9c0af3cb3fe1005f9f763e6a3f860f52af9741a4