powerpc/e500: Update compilation flags with core specific options

If CONFIG_E500 is enabled, the compilation flags are updated
specifying the target core -mcpu=e5500/e500mc/8540
Also remove -Wa,-me500, being incompatible with -mcpu=e5500/e6500
The assembler option is redundant if the -mcpu= flag is set.
The patch fixes the kernel compilation problem for e5500/e6500
when using gcc option -mcpu=e5500/e6500.

Signed-off-by: Catalin Udma <catalin.udma@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>