mmc: sd: optimize erase

To SD, there is no erase group, then the value erase_grp_size
will be default 1. When erasing SD blocks, the blocks will be
erased one by one, which is time consuming.

We use AU_SIZE as a group to speed up the erasing.

Erasing 4MB with a SD2.0 Card with AU_SIZE 4MB.
`time mmc erase 0x100000 0x2000`
time: 44.856 seconds (before optimization)
time: 0.335 seconds  (after optimization)

Signed-off-by: Peng Fan <[email protected]>
Cc: Jaehoon Chung <[email protected]>
Cc: Simon Glass <[email protected]>
Cc: Bin Meng <[email protected]>
Cc: Stefan Wahren <[email protected]>
Cc: Clemens Gruber <[email protected]>
Cc: Kever Yang <[email protected]>
Cc: Eric Nelson <[email protected]>
Cc: Stephen Warren <[email protected]>

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