dt-bindings: fsi: Add P10 OCC device documentation

Add the P10 compatible string.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201120010315.190737-2-joel@jms.id.au
Signed-off-by: Guenter Roeck <linux@roeck-us.net>