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: f3fb162 ("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>

Author: arndb

drivers/soc/Makefile

@@ -29,7 +29,6 @@ obj-y				+= renesas/
 obj-y				+= rockchip/
 obj-$(CONFIG_SOC_SAMSUNG)	+= samsung/
 obj-y				+= sifive/
-obj-$(CONFIG_SOC_STARFIVE)	+= starfive/
 obj-y				+= sunxi/
 obj-$(CONFIG_ARCH_TEGRA)	+= tegra/
 obj-y				+= ti/