rtc: sun6i: Make external 32k oscillator optional

Some boards, like OrangePi PC2 (H5), OrangePi Plus 2E (H3) and Tanix TX6
(H6) don't have external 32kHz oscillator. Till H6, it didn't really
matter if external oscillator was enabled because HW detected error and
fall back to internal one. H6 has same functionality but it's the first
SoC which have "auto switch bypass" bit documented and always enabled in
driver. This prevents RTC to work correctly if external crystal is not
present on board. There are other side effects - all peripherals which
depends on this clock also don't work (HDMI CEC for example).

Make clocks property optional. If it is present, select external
oscillator. If not, stay on internal.
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20200308135849.106333-2-jernej.skrabec@siol.netSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-sun6i.c

@@ -250,19 +250,17 @@ static void __init sun6i_rtc_clk_init(struct device_node *node,
 		writel(reg, rtc->base + SUN6I_LOSC_CTRL);
 	}
 
-	/* Switch to the external, more precise, oscillator */
+	/* Switch to the external, more precise, oscillator, if present */
+	if (of_get_property(node, "clocks", NULL)) {
 		reg |= SUN6I_LOSC_CTRL_EXT_OSC;
 		if (rtc->data->has_losc_en)
 			reg |= SUN6I_LOSC_CTRL_EXT_LOSC_EN;
+	}
 	writel(reg, rtc->base + SUN6I_LOSC_CTRL);
 
 	/* Yes, I know, this is ugly. */
 	sun6i_rtc = rtc;
 
-	/* Deal with old DTs */
-	if (!of_get_property(node, "clocks", NULL))
-		goto err;
-
 	/* Only read IOSC name from device tree if it is exported */
 	if (rtc->data->export_iosc)
 		of_property_read_string_index(node, "clock-output-names", 2,
@@ -279,11 +277,13 @@ static void __init sun6i_rtc_clk_init(struct device_node *node,
 	}
 
 	parents[0] = clk_hw_get_name(rtc->int_osc);
+	/* If there is no external oscillator, this will be NULL and ... */
 	parents[1] = of_clk_get_parent_name(node, 0);
 
 	rtc->hw.init = &init;
 
 	init.parent_names = parents;
+	/* ... number of clock parents will be 1. */
 	init.num_parents = of_clk_get_parent_count(node) + 1;
 	of_property_read_string_index(node, "clock-output-names", 0,
 				      &init.name);