mmc: new discard support at mmc v4.5

At mmc v4.5, it supports the DISCARD feature. It's different from trim and there's no check bit. currently it's only supported at v4.5.

In trim mode, the controller can perform the actual erase at a convenient time. however, In discard, The controller can perform partial or full the actual erase at a convenient time.

So in mmc v4.5, it's better to use the discard instead of trim.

Please give opinions or comments.

Thank you,
Kyungmin Park
---

index b8b1b7a..84f35a6 100644
@@ -146,6 +146,7 @@ extern int mmc_switch(struct mmc_card *, u8, u8, u8, unsigned int);
 #define MMC_ERASE_ARG		0x00000000
 #define MMC_SECURE_ERASE_ARG	0x80000000
 #define MMC_TRIM_ARG		0x00000001
+#define MMC_DISCARD_ARG		0x00000003
 #define MMC_SECURE_TRIM1_ARG	0x80000001
 #define MMC_SECURE_TRIM2_ARG	0x80008000