mpc512x: use common code for CSx configuration

Remove CSx configurations from board code and only define
required CSx macros in the board config file to configure
chip select windows and parameters.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Reinhard Arlt <reinhard.arlt@esd-electronics.com>
Cc: Wolfgang Denk <wd@denx.de>