powerpc/mpic: Add get_version API both for internal and external use

MPIC version is useful information for both mpic_alloc() and mpic_init().
The patch provide an API to get MPIC version for reusing the code.
Also, some other IP block may need MPIC version for their own use.
The API for external use is also provided.

Signed-off-by: Jia Hongtao <hongtao.jia@freescale.com>
Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>