mxc: add cscr register defintions

This adds macros to get CSCR upper, lower and additional registers.
These registers are needed to configure a chip select line. The offset
layouts of these Registers are identical on mx27 and mx31, hence we can
use the macros in generic way

Signed-off-by: Luotao Fu <l.fu@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>