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 <fabio.estevam@nxp.com>
Cc: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>