soc-family: fix SOC_FAMILY override order

soc-family: fix SOC_FAMILY override order

* the current order has SOC_FAMILY settings, which are generic
  settings for a group of devices, overriding the machine specific
  settings.  For example:

  KERNEL_DEVICETREE_ti33x = "xxxx"
  KERNEL_DEVICETREE_beaglebone = "yyyy"

  Should yield "yyyy" when building for the beaglebone because
  that is a more specific device than ti33x.  However, without this
  change the result is that the value is set to "xxxx" meaning the
  more generic setting overrides the more specific setting.

Signed-off-by: Chase Maupin <Chase.Maupin@ti.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>