dtc: Move tree checking code to checks.c

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> This patch moves the dtc code for checking the device tree its
> processing into a new checks.c.  The tree accessor functions from
> livetree.c which the checks use are exported and added to dtc.h.
> 
> Another small step towards a flexible checking architecture.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl