Revert "ARM: use -mcpu=cortex-a9 when targeting U8420 CPUs (Cortex-A9 based)"

This reverts commit d91315b342de356da5f95dbf6f21db3ccfa5a359.
ARM: use -mcpu=cortex-a9 when targeting U8500 CPUs (Cortex-A9 based)
This is done in order to force compliant GCC compiler to generate the new
sdiv/udiv instructions supported by Cortex-A9 architecture