edb93xx: Fix SDRAM initialization

edb93xx: Fix SDRAM initialization by issuing a precharge all command before
forcing the precharge and select mode register update mode before programming
the mode registers. Write to the SDRAM banks in order to force a precharge,
reading causes the edb93xx boards to hang

Signed-off-by: Matthias Kaehlcke <matthias@kaehlcke.net>