rockchip: clk: rk3368: add support for GMAC (SLCK_MAC) clock

To enable the GMAC on the RK3368, we need to set up the clocking
appropriately to generate a tx_clk for the MAC.

This adds an implementation that implements the use of the <&ext_gmac>
clock (i.e. an external 125MHz clock for RGMII provided by the PHY).
This is the clock setup used by the boards currently supported by
U-Boot (i.e. Geekbox, Sheep and RK3368-uQ7).

This includes the change from commit
 - rockchip: clk: rk3368: define GMAC_MUX_SEL_EXTCLK

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/arm/include/asm/arch-rockchip/cru_rk3368.h

@@ -89,6 +89,9 @@ enum {
 	MCU_CLK_DIV_SHIFT		= 0,
 	MCU_CLK_DIV_MASK		= GENMASK(4, 0),
 
+	/* CLKSEL43_CON */
+	GMAC_MUX_SEL_EXTCLK             = BIT(8),
+
 	/* CLKSEL51_CON */
 	MMC_PLL_SEL_SHIFT		= 8,
 	MMC_PLL_SEL_MASK		= GENMASK(9, 8),

drivers/clk/rockchip/clk_rk3368.c

@@ -338,6 +338,19 @@ static ulong rk3368_ddr_set_clk(struct rk3368_cru *cru, ulong set_rate)
 }
 #endif
 
+#if CONFIG_IS_ENABLED(GMAC_ROCKCHIP)
+static ulong rk3368_gmac_set_clk(struct rk3368_cru *cru,
+				 ulong clk_id, ulong set_rate)
+{
+	/*
+	 * This models the 'assigned-clock-parents = <&ext_gmac>' from
+	 * the DTS and switches to the 'ext_gmac' clock parent.
+	 */
+	rk_setreg(&cru->clksel_con[43], GMAC_MUX_SEL_EXTCLK);
+	return set_rate;
+}
+#endif
+
 static ulong rk3368_clk_set_rate(struct clk *clk, ulong rate)
 {
 	struct rk3368_clk_priv *priv = dev_get_priv(clk->dev);
@@ -356,10 +369,12 @@ static ulong rk3368_clk_set_rate(struct clk *clk, ulong rate)
 		ret = rk3368_mmc_set_clk(clk, rate);
 		break;
 #endif
+#if CONFIG_IS_ENABLED(GMAC_ROCKCHIP)
 	case SCLK_MAC:
-		/* nothing to do, as this is an external clock */
-		ret = rate;
+		/* select the external clock */
+		ret = rk3368_gmac_set_clk(priv->cru, clk->id, rate);
 		break;
+#endif
 	default:
 		return -ENOENT;
 	}