MAINTAINERS: Move myself from SPRD Maintainer to Reviewer

I've resigned from Unisoc (formerly Spreadtrum) and cannot access
internal HW specifications any more, also cannot commit much
time for maintaining sprd code, especially DTS.

While I'm happy to help review the sprd drivers I wrote before.

Signed-off-by: Chunyan Zhang <zhangchunyan@iscas.ac.cn>
Link: https://lore.kernel.org/r/20240705091133.3308539-1-zhangchunyan@iscas.ac.cn


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