ARM: dts: da850-evm: use phandles to extend nodes

Many node labels in the device tree (like serial0, serial1, etc) are being
redefined, so let's modernize the device tree by using phandles to
extend the existing nodes.  This helps reduce the whitespace.

Signed-off-by: Adam Ford <aford173@gmail.com>
[nsekhar@ti.com: drop tps6507x related changes]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>