[CORE] vmm_devtree: Return const pointer from vmm_devtree_attrval() API

The vmm_devtree_attrval() API returns pointer to attribute value of
a device tree node. This pointer has to be const to avoid accidental
updates to attribute value thereby reducing potential buggy use of
vmm_devtree_attrval() API.

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