serial: stm32x7: migrate serial struct to driver

This allow to remove include/dm/platform_data/serial_stm32x7.h
which was included in the past by stm32x7 driver and by
stm32f746-disco.c board file.
Since patch 42bf5e7c27 "serial: stm32f7: add device tree support"
this file is no more needed in board file.

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 facfdba..42b05f1 100644
--- a/drivers/serial/serial_stm32x7.h
+++ b/drivers/serial/serial_stm32x7.h
@@ -22,6 +22,11 @@
 	u32 tx_dr;
 };
 
+/* Information about a serial port */
+struct stm32x7_serial_platdata {
+	struct stm32_usart *base;  /* address of registers in physical memory */
+	unsigned int clock;
+};
 
 #define USART_CR1_OVER8			(1 << 15)
 #define USART_CR1_TE			(1 << 3)