powerpc: gamecube_defconfig: Enable the RTC driver

This selects the rtc-gamecube driver, which provides a real-time clock
on this platform.

Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20211215175501.6761-5-linkmauve@linkmauve.fr