The existing code assumes the SDRAM row refresh period should always
be 15.6 us. This is not always true, and indeed on the ATNGW100, the
refresh rate should really be 7.81 us.

Add a refresh_period member to struct sdram_info and initialize it
properly for both ATSTK1000 and ATNGW100. Out-of-tree boards will
panic() until the refresh_period member is updated properly.

Big thanks to Gerhard Berghofer for pointing out this issue.
Signed-off-by: NHaavard Skinnemoen <hskinnemoen@atmel.com>