At 133 MHz the current SDRAM refresh rate is too fast
(measured 4 * 1.17 us).
CFG_MAMR_PTA changes from 39 to 97. This result
in a refresh rate of 4 * 7.8 us at the default clock
50 MHz. At 133 MHz the value will be then 4 * 2.9 us.
This is a compromise until a new method is found to
adjust the refresh rate.
Signed-off-by: NMartin Krause <martin.krause@tqs.de>