ARM: imx6: Adapt device tree selection in DH board file

Before this commit device tree selection could rely solely on
differentiating the iMX6 processor variant Q and DL. After adding two new
carrier boards, the DRC02 and the picoITX, the interchangeability of SoMs
makes this approach infeasible.

It is now required to specify the carrier board (dhcom-drc02,
dhcom-picoitx or dhcom-pdk2) at compile time using
CONFIG_DEFAULT_DEVICETREE. The SoM is determined at runtime as before.

Signed-off-by: Philip Oberfichtner <pro@denx.de>