powerpc/5200: Use the gpt* labels to simplify mpc5200 dts files

The DTC labels feature allows a dts file to reference a node without
having to reproduce the entire node hierarchy above it. We can use this
to simplify the MPC5200 board dts files by referencing the gpt nodes by
label.

Cc: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
[agust: fixed gpt7 phandle in the csi node of o2d.dtsi]
Signed-off-by: Anatolij Gustschin <agust@denx.de>