mvgbe: enable configurability of PORT_SERIAL_CONTROL_VALUE

This allows this configuration to be defined differently for some
boards that request it.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Holger Brunck <holger.brunck@keymile.com>
cc: Heiko Schocher <hs@denx.de>
cc: Wolfgang Denk <wd@denx.de>
cc: Detlev Zundel <dzu@denx.de>
cc: Prafulla Wadaskar <prafulla@marvell.com>

drivers/net/mvgbe.h

@@ -84,6 +84,7 @@
 	MVGBE_TX_BURST_SIZE_16_64BIT)
 
 /* Default port serial control value */
+#ifndef PORT_SERIAL_CONTROL_VALUE
 #define PORT_SERIAL_CONTROL_VALUE		( \
 	MVGBE_FORCE_LINK_PASS			| \
 	MVGBE_DIS_AUTO_NEG_FOR_DUPLX		| \
@@ -101,6 +102,7 @@
 	MVGBE_CLR_EXT_LOOPBACK			| \
 	MVGBE_SET_FULL_DUPLEX_MODE		| \
 	MVGBE_DIS_FLOW_CTRL_TX_RX_IN_FULL_DUPLEX)
+#endif
 
 /* Tx WRR confoguration macros */
 #define PORT_MAX_TRAN_UNIT	0x24	/* MTU register (default) 9KByte */