MIPS: mscc: Prepare configuration to handle more SoCs

Ocelot belongs to a family of SoC named the VCore III. In order to add
these new Soc, use the new symbol SOC_VCOREIII instead of a one
dedicated to Ocelot.

In order to avoid regression on driver building, the MSCC_OCELOT
configuration symbol is kept until the driver will be converted.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>