mmc: remove hc_wp_grp_size from struct mmc if not needed

hc_wp_grp_size is needed only if hardware partitionning is used.
On ARM removing it saves about 30 bytes of code space.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
diff --git a/drivers/mmc/mmc.c b/drivers/mmc/mmc.c
index c1ba57f..53c8191 100644
--- a/drivers/mmc/mmc.c
+++ b/drivers/mmc/mmc.c
@@ -2098,9 +2098,11 @@
 			* (erase_gmul + 1);
 	}
 #endif
+#if CONFIG_IS_ENABLED(MMC_HW_PARTITIONING)
 	mmc->hc_wp_grp_size = 1024
 		* ext_csd[EXT_CSD_HC_ERASE_GRP_SIZE]
 		* ext_csd[EXT_CSD_HC_WP_GRP_SIZE];
+#endif
 
 	mmc->wr_rel_set = ext_csd[EXT_CSD_WR_REL_SET];