checks: Pass boot_info instead of root node

As preparation for overlay support we need to pass the boot info
parameter instead of the root node to each check method.

The root node can be retrieved by accessing boot info's dt member.

No other functional changes are made.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>