[CORE] vmm_devtree: Add attrval_string_index() and iterate_matching() APIs

This patch adds two more useful device tree APIs:
1. vmm_devtree_attrval_string_index() - Access string based on index from
   list of strings
2. vmm_devtree_iterate_matching() - Iterate over all matching nodes under
   given subtree of device tree nodes

The above two newly added APIs will be used for common device tree based
clocking framework.

Signed-off-by: Anup Patel <anup@brainfault.org>