[CORE] vmm_devtree: Store attribute values in big-endian format

Currently, we store attribute values in native-endian format. The idea
behind doing so was to avoid unnecassary endian swapping when reading
or writing attribute values. The Linux OF framework on the other-hand
keeps attribute values as big-endian because FDT is big-endian.

This patch updates vmm_devtree to store attribute values in big-endian
format. This will help us as follows:
1. Make vmm_devtree more Linux compatible
2. Ease driver porting from Linux because lot of Linux drivers
directly access attribute values assuming big-endian format
3. No need for endian related swaping of literal values when
populating device tree from FDT

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