omap3: mem: Define and use common macros

Define common macros to arrive at the values of registers
SDRC_ACTIM_CTRLA and SDRC_ACTIM_CTRLB for different memory
types.

This doesn't make any real change in the execution but
helps readability.

Signed-off-by: Sanjeev Premi <premi@ti.com>
Cc: Sandeep Paulraj <s-paulraj@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>