ARM: Makefile: Use -mcpu=cortex-a9 when targeting Exynos4 CPUs

Enable compiler optimizations specific to the Cortex-A9
processor when targeting Exynos4 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'.

Signed-off-by: Zhao Wei Liew <zhaoweiliew@gmail.com>
ghost: Default to NOOP scheduler

Signed-off-by: Zhao Wei Liew <zhaoweiliew@gmail.com>