serial: stm32x7: add clk_get_rate() support

Replace proprietary clock_get() by clk_get_rate()
The stm32x7 serial driver is now "generic" and can be used
by other STM32 SoCs.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Acked-by: Vikas MANOCHA <vikas.manocha@st.com>
diff --git a/drivers/serial/serial_stm32x7.h b/drivers/serial/serial_stm32x7.h
index 42b05f1..9fe37af 100644
--- a/drivers/serial/serial_stm32x7.h
+++ b/drivers/serial/serial_stm32x7.h
@@ -25,7 +25,7 @@
 /* Information about a serial port */
 struct stm32x7_serial_platdata {
 	struct stm32_usart *base;  /* address of registers in physical memory */
-	unsigned int clock;
+	unsigned long int clock_rate;
 };
 
 #define USART_CR1_OVER8			(1 << 15)