ARMV7: OMAP3: Fix bug in get_sdr_cs_offset()

This patch fixes a typo in the routine to calculate the cs offset
based upon the contents of the SDRC cs_cfg register.  This function
mistakenly shifts the CS1STARTLOW field 17 bits right instead of
17 bits left.

This hasn't been an issue to date because all OMAP3 boards currently
are configured to have zeros in this field.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Steve Sakoman <steve.sakoman@linaro.org>