ARM: bcm2708: use correct device-tree node names for serial

Upstream mandates the use of "serial" as name inside the device-tree.
Note that this does not affect the use of the alias - there
uart0 and uart1 are permissible.

This also fixed the patch references inside the overlays for the rpi3.

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>