sanitycheck: Add functions to query device tree for filters

Add the following functions to allow filtering based on device tree

dt_compat_enabled(compat) - Returns true if a device tree node
compatible matches 'compat' and the node is enabled.

dt_alias_exists(alias) - Returns true if a device tree node exists with
'alias' and the node is enabled.

dt_compat_enabled_with_alias - Returns true if a device tree node
compatible matches 'compat' and the node has 'alias' and the node is
enabled.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>