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.

This function had been previously added but was removed by commit
5e86bfde

 ("powerpc/mpic: remove unused functions") due to the
lack of a user.  This function will be used by "powerpc/mpic: Add
get_version API both for internal and external use".

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