rtc: add dm_rtc_read helper and ->read method

Some users may want to read multiple consecutive 8-bit
registers. Instead of each caller having to implement the loop,
provide a dm_rtc_read() helper. Also, allow a driver to provide a
->read method, which can be more efficient than reading one register
at a time.

Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>

include/rtc.h

@@ -55,6 +55,18 @@ struct rtc_ops {
 	 */
 	int (*reset)(struct udevice *dev);
 
+	/**
+	 * read() - Read multiple 8-bit registers
+	 *
+	 * @dev:	Device to read from
+	 * @reg:	First register to read
+	 * @buf:	Output buffer
+	 * @len:	Number of registers to read
+	 * @return 0 if OK, -ve on error
+	 */
+	int (*read)(struct udevice *dev, unsigned int reg,
+		    u8 *buf, unsigned int len);
+
 	/**
 	 * read8() - Read an 8-bit register
 	 *
@@ -109,6 +121,17 @@ int dm_rtc_set(struct udevice *dev, struct rtc_time *time);
  */
 int dm_rtc_reset(struct udevice *dev);
 
+/**
+ * dm_rtc_read() - Read multiple 8-bit registers
+ *
+ * @dev:	Device to read from
+ * @reg:	First register to read
+ * @buf:	Output buffer
+ * @len:	Number of registers to read
+ * @return 0 if OK, -ve on error
+ */
+int dm_rtc_read(struct udevice *dev, unsigned int reg, u8 *buf, unsigned int len);
+
 /**
  * rtc_read8() - Read an 8-bit register
  *