MAINTAINERS: thead: update Maintainer

Due to personal reasons, I can't maintain T-Head SoCs any more. At the
same time, I would nominate Drew Fustini as Maintainer. Drew contributed
the sdhci support of TH1520 in the past, and is working on the clk
parts. I believe he will look after T-Head SoCs.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Acked-by: Drew Fustini <drew@pdp7.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>