ath79: create WNDR3700 series .dtsi and adjust WNDR3800

ath79: create WNDR3700 series .dtsi and adjust WNDR3800

Prepare for addition of WNDR3700 and WNDR3700v2 by
separating the common parts into wndr3700.dtsi and
leaving just the device-specific things into wndr3800.dts

The three routers are identical except
  * device IDs
  * WNDR3700 (v1) has only 8 MB flash, while others have 16 MB.
    Partition structure needs to be defined for each device.
  * (WNDR3800 has 128 MB RAM, but RAM size is not in DTS)

Also separate the common parts of the image recipe.
(Drop also the initramfs recipe.)

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>