scsi: ufs: core: Allow UFS host drivers to override the sg entry size

Modify the UFSHCD core to allow 'struct ufshcd_sg_entry' to be
variable-length. The default is the standard length, but variants can
override ufs_hba::sg_entry_size with a larger value if there are
vendor-specific fields following the standard ones.

This is needed to support inline encryption with ufs-exynos (FMP).

Cc: Eric Biggers <ebiggers@google.com>
Reviewed-by: Avri Altman <avri.altman@wdc.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
[ bvanassche: edited commit message and introduced CONFIG_SCSI_UFS_VARIABLE_SG_ENTRY_SIZE ]
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>