arm: use -mcpu=cortex-a15 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-a15' is a superset of 'armv7-a'.

Change-Id: I9154a446860b4f4c2a3fb9fb12fb23feec2c1b0e