toradex: imx6*: fix reset using wdt-reboot driver

commit 68dcbdd5 ("ARM: imx: Add weak default reset_cpu()")
introduced a regression that 'reset' command unable to reset
imx6 and imx6ull based Toradex's modules in the u-boot.

This change enables a watchdog-based sysreset driver for
Toradex's colibri-imx6, colibri-imx6ull (with nand and emmc)
and apalis-imx6 which solves the reset command regression on
these modules.

Fixes: 68dcbdd5 ("ARM: imx: Add weak default reset_cpu()")
Signed-off-by: Parth Pancholi <parth.pancholi@toradex.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>