[POWERPC] Update device tree binding for mpc5200 gpt

Add 'fsl,' prefix to 'compatible' property for gpt nodes.
Add 'fsl,' prefix to empty, GPT0 specific 'has-wdt' property.
The fsl, prefix is being added to better match the convention of prefixing
manufacturer specific properties and values with the vendors name.

Signed-off-by: Marian Balakowicz <m8@semihalf.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>