ARM: mvebu: improve ll_set_cpu_coherent

ll_set_cpu_coherent does two things to set the coherency of a CPU:

- Add CPU to SMP group
- Enable coherency on CPU

Only the second part is needed when coming back from a CPU Idle. This
commit allows to choose to do the first part or not.

Until now the calling function have to know the base address of the
coherency registers. This commit doesn't expose anymore this address,
ans replace the argument by a flag to indicate if we have to use the
physical or the virtual address.

set_cpu_coherent is always use on the current cpu, so instead of
passing the cpu id as argument, set_cpu_coherent() can find it by
itself.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>