The object file is referenced by the Makefile, but the C file was not added in the commit.
make[3]: *** No rule to make target 'drivers/char/rk3568-gpiomem.o', needed by 'drivers/char/built-in.a'. Stop.
make[2]: *** [scripts/Makefile.build:500: drivers/char] Error 2
make[1]: *** [scripts/Makefile.build:500: drivers] Error 2
make: *** [Makefile:2012: .] Error 2