soc: microchip: make mpfs_sys_controller_put static

dsafsdfd0054 ("soc: add microchip polarfire soc system controller")
incorrectly exported mpfs_sys_controller_put. Remove the export and make
the function static instead. This fixes the "no previous prototype
for 'mpfs_sys_controller_put'" warning spotted by the kernel test robot.

Fixes: d0054a47 ("soc: add microchip polarfire soc system controller")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Conor Dooley <mail@conchuod.ie>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>