MFC r307943-r307944, r308698

r307943 by andrew:
Remove the need for the delay to be zero when MULTIDELAY is undefined,
it may be useful to only enable this in some configs.

Sponsored by: ABT Systems Ltd

r307944 by andrew:
Add MULTIDELAY support to the am335x dmtimer. This will be useful for
testing Cortex-A8 support in GENERIC.

Sponsored by: ABT Systems Ltd

r308698 by loos:
After r308533, the platform compatible string must be an exact match.

Use "ti,am33xx" instead of "ti,am335x", which gives an exact match in every
DTS we support.

This fixes the boot on TI SoCs after r308533.

Suggested by: gonzo
Sponsored by: Rubicon Communications, LLC (Netgate)

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add a MULTIDELAY option to allow the ARM kernel to have multiple DELAY
implementations. Early in the boot the kernel will use an approximate,
however after the timer has been probed it will switch to a more accurate
implementation.

Reviewed by: manu
Sponsored by: ABT Systems Ltd
Differential Revision: https://reviews.freebsd.org/D5762

Add FDT_PLATFORM_DEF2 for when there are multiple platforms needing to use
the same platform methods.

Add the start of the ARM platform code. This is based on the PowerPC
platform code, it is expected these will be merged in the future when the
ARM code is more complete.

Until more boards can be tested only use this with the Raspberry Pi and
rrename the functions on the other SoCs.

Reviewed by: ian@