dm: core: Add functions to read 64-bit dt properties

This patch adds functions dev_read_u64_default & dev_read_u64
to read unsigned 64-bit values from devicetree.

Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>