cfg: add basic support of Freescale i.MX6 series targets

This change adds a simple target configuration for Freescale
single/dual/quad core i.MX6 SoCs, only one core is configured by default.

Change-Id: I853dd27f4c6765b7f731be2ddea82e85d496c6a4
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-on: http://openocd.zylin.com/1135
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>