dm3730-am3715-evm: split machine type into two

dm3730-am3715-evm: split machine type into two

* Split the dm3730-am3715-evm machine type into two separate
  machines.  These are dm37x-evm and am37x-evm.
* This split will allow for being able to distiguish different
  MACHINE_FEATURES in the future such as whether the machine has
  a DSP or not.  This will allow for cleaner task creation.
* In order to be able to choose the proper collateral and software
  such as documentation and codecs we need machine types that
  differentiate product lines (i.e. AM vs DM) and product
  families (i.e. 37x vs 35x).  This change allows enough
  differentiation to chose proper collateral without
  requiring a machine type for every derivative chip.

Signed-off-by: Chase Maupin <chase.maupin@ti.com>
Acked-by: Denys Dmytriyenko <denys@ti.com>
Signed-off-by: Denys Dmytriyenko <denis@denix.org>