MAINTAINERS: thead: add git tree

Add git tree for T-Head device tree files and, in the future, drivers for
the T-Head TH1520 SoC.

Signed-off-by: Drew Fustini <drew@pdp7.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240704180913.2516959-1-drew@pdp7.com


Signed-off-by: Arnd Bergmann <arnd@arndb.de>