serial: 8250_em: Use pseudo offset for UART_FCR

UART_FCR shares the same offset with UART_IIR. We cannot use UART_FCR
in serial8250_em_serial_in() as it overlaps with UART_IIR.

Define UART_FCR_EM macro with a high value to avoid overlapping
with existing UART_* register defines and define another macro
UART_FCR_EM_HW for the real offset.

Use these macros in serial8250_em_serial{_in/_out} function to
read/write FCR register.
Suggested-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/20230227114152.22265-7-biju.das.jz@bp.renesas.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_em.c

@@ -19,6 +19,13 @@
 #define UART_DLL_EM 9
 #define UART_DLM_EM 10
 
+/*
+ * A high value for UART_FCR_EM avoids overlapping with existing UART_*
+ * register defines. UART_FCR_EM_HW is the real HW register offset.
+ */
+#define UART_FCR_EM 0x10003
+#define UART_FCR_EM_HW 3
+
 struct serial8250_em_priv {
 	int line;
 };
@@ -29,12 +36,15 @@ static void serial8250_em_serial_out(struct uart_port *p, int offset, int value)
 	case UART_TX: /* TX @ 0x00 */
 		writeb(value, p->membase);
 		break;
-	case UART_FCR: /* FCR @ 0x0c (+1) */
 	case UART_LCR: /* LCR @ 0x10 (+1) */
 	case UART_MCR: /* MCR @ 0x14 (+1) */
 	case UART_SCR: /* SCR @ 0x20 (+1) */
 		writel(value, p->membase + ((offset + 1) << 2));
 		break;
+	case UART_FCR:
+	case UART_FCR_EM:
+		writel(value, p->membase + (UART_FCR_EM_HW << 2));
+		break;
 	case UART_IER: /* IER @ 0x04 */
 		value &= 0x0f; /* only 4 valid bits - not Xscale */
 		fallthrough;
@@ -55,6 +65,8 @@ static unsigned int serial8250_em_serial_in(struct uart_port *p, int offset)
 	case UART_MSR: /* MSR @ 0x1c (+1) */
 	case UART_SCR: /* SCR @ 0x20 (+1) */
 		return readl(p->membase + ((offset + 1) << 2));
+	case UART_FCR_EM:
+		return readl(p->membase + (UART_FCR_EM_HW << 2));
 	case UART_IER: /* IER @ 0x04 */
 	case UART_IIR: /* IIR @ 0x08 */
 	case UART_DLL_EM: /* DLL @ 0x24 (+9) */