MAINTAINERS: Set myself as Goldfish RTC maintainer

While Goldfish platform is dusted, the RTC driver remains
valuable for us.

I'm volunteering to maintain goldfish RTC driver onward.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Miodrag Dinic <Miodrag.Dinic@syrmia.com>
Link: https://lore.kernel.org/r/20201114130921.651882-3-jiaxun.yang@flygoat.com