kirkwood: added common config file mv-common.h

It is observed that, in most of the board configs the code is being
duplicated, also for any common change all board files needs update
This issue was under discussion from long on mailing list and we
converge on introducing common config file.

With this patch-
1. Total Kirkwood specific configuration code is reduced by 210 lines
2. All common configuration can be shared by multiple boards
3. Easy to manage common updates like ARM relocation changes

mv-common.h file is added to include/configs/
It contains all common configuration supported for all Kirkwood boards
The respective board configs are updated for its usage

Build tested for guruplug, mv88f6281gtw_ge, openrd_base,
rd6281a and sheevaplug
Binary execution tested for sheevaplug

Todo:
1. Other custom Kirkwood boards to be synced
2. The support to be extended for Orion5X based boards

Signed-off-by: Prafulla Wadaskar <prafulla@marvell.com>