MAINTAINERS: setup support for SpacemiT SoC tree

Add myself as maintainer of SpacemiT's SoC tree, which
suggested by Conor [1].

Link: https://lore.kernel.org/all/20241018234615-GYA2124001@gentoo/ [1]
Signed-off-by: Yixun Lan <[email protected]>

Author: dlan17

MAINTAINERS

@@ -20179,6 +20179,15 @@ F:	drivers/perf/riscv_pmu.c
 F:	drivers/perf/riscv_pmu_legacy.c
 F:	drivers/perf/riscv_pmu_sbi.c
 
+RISC-V SPACEMIT SoC Support
+M:	Yixun Lan <[email protected]>
+L:	[email protected]
+S:	Maintained
+T:	git https://github.com/spacemit-com/linux
+F:	arch/riscv/boot/dts/spacemit/
+N:	spacemit
+K:	spacemit
+
 RISC-V THEAD SoC SUPPORT
 M:	Drew Fustini <[email protected]>
 M:	Guo Ren <[email protected]>