pico-imx7d: Correct uart clock root

Correct uart clock root ID. Incorrect ID may result the
clock is gated because rate value 0 is returned in
imx_get_uartclk()

The ID can be ignored if CONFIG_SKIP_LOWLEVEL_INIT is not enabled
because init_clk_uart() will enable all uart clocks in that case.

Signed-off-by: Jun Nie <jun.nie@linaro.org>
diff --git a/arch/arm/mach-imx/mx7/clock.c b/arch/arm/mach-imx/mx7/clock.c
index 8cda71c..e364b16 100644
--- a/arch/arm/mach-imx/mx7/clock.c
+++ b/arch/arm/mach-imx/mx7/clock.c
@@ -53,7 +53,7 @@
 
 u32 imx_get_uartclk(void)
 {
-	return get_root_clk(UART1_CLK_ROOT);
+	return get_root_clk(UART_CLK_ROOT);
 }
 
 u32 imx_get_fecclk(void)