rtc: fall back to ->{read, write} if ->{read, write}8 are not provided

Similar to how the dm_rtc_{read,write} functions fall back to using
the {read,write}8 methods, do the opposite in the rtc_{read,write}8
functions.

This way, each driver only needs to provide either ->read8 or ->read
to make both rtc_read8() and dm_rtc_read() work - without this, a
driver that provides ->read() would most likely just duplicate the
logic here for implementing a ->read8() method in term of its ->read()
method. The same remarks of course apply to the write case.
Reviewed-by: NSimon Glass <sjg@chromium.org>
Reviewed-by: NHeiko Schocher <hs@denx.de>
Signed-off-by: NRasmus Villemoes <rasmus.villemoes@prevas.dk>

drivers/rtc/rtc-uclass.c

@@ -83,9 +83,17 @@ int rtc_read8(struct udevice *dev, unsigned int reg)
 	struct rtc_ops *ops = rtc_get_ops(dev);
 
 	assert(ops);
-	if (!ops->read8)
-		return -ENOSYS;
-	return ops->read8(dev, reg);
+	if (ops->read8)
+		return ops->read8(dev, reg);
+	if (ops->read) {
+		u8 buf[1];
+		int ret = ops->read(dev, reg, buf, 1);
+
+		if (ret < 0)
+			return ret;
+		return buf[0];
+	}
+	return -ENOSYS;
 }
 
 int rtc_write8(struct udevice *dev, unsigned int reg, int val)
@@ -93,9 +101,14 @@ int rtc_write8(struct udevice *dev, unsigned int reg, int val)
 	struct rtc_ops *ops = rtc_get_ops(dev);
 
 	assert(ops);
-	if (!ops->write8)
-		return -ENOSYS;
-	return ops->write8(dev, reg, val);
+	if (ops->write8)
+		return ops->write8(dev, reg, val);
+	if (ops->write) {
+		u8 buf[1] = { val };
+
+		return ops->write(dev, reg, buf, 1);
+	}
+	return -ENOSYS;
 }
 
 int rtc_read16(struct udevice *dev, unsigned int reg, u16 *valuep)