am33xx,ddr3: fix ddr3 sdram configuration

This patch fixes the DDR3 initialization procedure in
order to comply with DDR3 standard. A 500 us delay is specified
between the DDR3 reset and clock enable signal. Until now,
this delay was not respected. Some DDR3 chips don't bother
but the bigger the RAM becomes the more likely it seems that
this delay is needed. We observed that DRAM > 256 MB from
the manufacturer Samsung have an issue when the specification
is not respected.

Changes:

1) Add a 1 ms wait for L3 timeout error trigger

2) Don't delay DDR3 initialization
Bit 31 of emif_sdram_ref_ctrl shouldn't be set because his
suppresses the initialization of DDR3

Signed-off-by: Samuel Egli <samuel.egli@siemens.com>
Reviewed-by: James Doublesin <doublesin@ti.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Roger Meier <r.meier@siemens.com>
Cc: Heiko Schocher <hs@denx.de>