soc: starfive: remove stale Makefile entry

After the only starfive driver was moved out to the genpd subsystem, we get
a build failure:

scripts/Makefile.build:41: drivers/soc/starfive/Makefile: No such file or directory

Fixes: f3fb1629 ("soc: starfive: Move the power-domain driver to the genpd dir")
Reported=by: kernel test robot <lkp@intel.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>