mx6slevk: imximage.cfg: update to fix tINIT3 and tIH-CA violations

mx6slevk: imximage.cfg: update to fix tINIT3 and tIH-CA violations

Having had a similar board and memory part under logic analyzer, a
tINIT3 violation was measured.  The fix was involved keeping tXPR and
SDE_to_RST at the power-on defaults and setting RST_to_CKE the JEDEC
value for LPDDR2.  There was also a tIH-CA violation and this was
resolved by writing the default value in rather than what the script
here uses.

Cc: Fabio Estevam <[email protected]>
Cc: Peng Fan <[email protected]>
Signed-off-by: Tom Rini <[email protected]>
Reviewed-by: Fabio Estevam <[email protected]>