Allow Exynos UFS driver to build as a module.  This patch fixes the
followin build issue reported by kernel build robot.

drivers/scsi/ufs/ufs-exynos.o: in function `exynos_ufs_probe':
drivers/scsi/ufs/ufs-exynos.c:1231: undefined reference to `ufshcd_pltfrm_init'
drivers/scsi/ufs/ufs-exynos.o: in function `exynos_ufs_pre_pwr_mode':
drivers/scsi/ufs/ufs-exynos.c:635: undefined reference to `ufshcd_get_pwr_dev_param'
drivers/scsi/ufs/ufs-exynos.o:undefined reference to `ufshcd_pltfrm_shutdown'
drivers/scsi/ufs/ufs-exynos.o:undefined reference to `ufshcd_pltfrm_suspend'
drivers/scsi/ufs/ufs-exynos.o:undefined reference to `ufshcd_pltfrm_resume'
drivers/scsi/ufs/ufs-exynos.o:undefined reference to `ufshcd_pltfrm_runtime_suspend'
drivers/scsi/ufs/ufs-exynos.o:undefined reference to `ufshcd_pltfrm_runtime_resume'
drivers/scsi/ufs/ufs-exynos.o:undefined reference to `ufshcd_pltfrm_runtime_idle'

Link: https://lore.kernel.org/r/20200620173232.52521-1-alim.akhtar@samsung.com
Fixes: 55f4b1f7 ("scsi: ufs: ufs-exynos: Add UFS host support for Exynos SoCs")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>