[CORE] vmm_devtree: Add phandle parsing APIs

This patch primarily adds phandle parsing APIs to
device tree framework. The phandle APIs will be
used to implement common device tree based clocking
framework in drivers. In future, it will be also
used for GPIO and PINCTRL driver frameworks.

In addition to above, this patch also does:
1. Reorganize functions in vmm_devtree based on
   dependency on each other.
2. Add vmm_devtree_attrval_match_string() and
   vmm_devtree_attrval_count_strings() APIs

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