MFC r310343-r310344

r310343:
[iMX6] Fix build for SSI driver and add dependency for SDMA driver

• Pass correct pointer to OF_getencprop
• Check the size of "dmas" property
• Add dependency on sdma driver

Reviewed by: br
Differential Revision: https://reviews.freebsd.org/D8873

r310344:
[iMX6] Fix SDMA driver build

• Place const modifiers where required
• Make sure sdma device is attahched before consumers like SSI

Reviewed by: br
Differential Revision: https://reviews.freebsd.org/D8874