atmel-mci: support multiple mmc slots

The Atmel MCI controller can drive multiple cards through separate sets
of pins, but only one at a time. This patch adds support for
multiplexing access to the controller so that multiple card slots can be
used as if they were hooked up to separate mmc controllers.

The atmel-mci driver registers each slot as a separate mmc_host. Both
access the same common controller state, but they also have some state
on their own for card detection/write protect handling, and separate
shadows of the MR and SDCR registers.

When one of the slots receives a request from the mmc core, the common
controller state is checked. If it's idle, the request is submitted
immediately. If not, the request is added to a queue. When a request is
done, the queue is checked and if there is a queued request, it is
submitted before the completion callback is called.

This patch also includes a few cleanups and fixes, including a locking
overhaul. I had to change the locking extensively in any case, so I
might as well try to get it right. The driver no longer takes any
irq-safe locks, which may or may not improve the overall system
performance.

This patch also adds a bit of documentation of the internal data
structures.
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>

arch/avr32/include/asm/atmel-mci.h

 #ifndef __ASM_AVR32_ATMEL_MCI_H
 #define __ASM_AVR32_ATMEL_MCI_H
 
+#define ATMEL_MCI_MAX_NR_SLOTS	2
+
 /**
  * struct mci_slot_pdata - board-specific per-slot configuration
  * @bus_width: Number of data lines wired up the slot
@@ -11,6 +13,10 @@
  * set to 0. The other fields are ignored in this case.
  *
  * Any pins that aren't available should be set to a negative value.
+ *
+ * Note that support for multiple slots is experimental -- some cards
+ * might get upset if we don't get the clock management exactly right.
+ * But in most cases, it should work just fine.
  */
 struct mci_slot_pdata {
 	unsigned int		bus_width;
@@ -23,7 +29,7 @@ struct mci_slot_pdata {
  * @slot: Per-slot configuration data.
  */
 struct mci_platform_data {
-	struct mci_slot_pdata	slot[2];
+	struct mci_slot_pdata	slot[ATMEL_MCI_MAX_NR_SLOTS];
 };
 
 #endif /* __ASM_AVR32_ATMEL_MCI_H */