ARM: dts: bcm283x: Move the BCM2837 DT contents from arm64 to arm.

BCM2837 is somewhat unusual in that we build its DT on both arm32 and
arm64.  Most devices are being run in arm32 mode.

Having the body of the DT for 2837 separate from 2835/6 has been a
source of pain, as we often need to make changes that span both
directories simultaneously (for example, the thermal changes for 4.13,
or anything that changes the name of a node referenced by '&' from
board files).  Other changes are made more complicated than they need
to be, such as the SDHOST enabling, because we have to split a single
logical change into a 283[56] half and a 2837 half.

To fix this, make the stub board include file live in arm64 instead of
arm32, and keep all of BCM283x's contents in arm32.  From here on, our
changes to DT contents can be submitted through a single tree.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>