MIPS: mscc: Fix configuration name for ocelot legacy boards

Ocelots is supported by the generic MIPS build so make it clears that
LEGACY_BOARD_OCELOT is only needed for legacy boards which didn't have
bootloader supporting device tree.

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>