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>
diff --git a/arch/arm/include/asm/arch-rockchip/cru_rk3368.h b/arch/arm/include/asm/arch-rockchip/cru_rk3368.h
index 21f11e0..2b1197f 100644
--- a/arch/arm/include/asm/arch-rockchip/cru_rk3368.h
+++ b/arch/arm/include/asm/arch-rockchip/cru_rk3368.h
@@ -89,6 +89,9 @@
 	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),