libpayload: usb: Add interval attribute to endpoints

Read bInterval from endpoint descriptors and store it in our endpoint_t
struct. The interval is encoded dependently on the device' speed and the
endpoint's type. Therefore, it will be normalized to the binary logarithm
of the number of microframes, i.e.
  t = 125us * 2^interval

The interval attribute will be used in the xHCI driver.

Change-Id: I65a8eda6145faf34666800789f0292e640a8141b
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: http://review.coreboot.org/3449
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

payloads/libpayload/include/usb/usb.h

@@ -91,6 +91,8 @@ typedef struct {
 	int toggle;
 	int maxpacketsize;
 	endpoint_type type;
+	int interval; /* expressed as binary logarithm of the number
+			 of microframes (i.e. t = 125us * 2^interval) */
 } endpoint_t;
 
 enum { FULL_SPEED = 0, LOW_SPEED = 1, HIGH_SPEED = 2, SUPER_SPEED = 3 };