Convert CONFIG_FSL_SERDES to Kconfig

This converts the following to Kconfig:
   CONFIG_FSL_SERDES

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/MPC837XERDB_defconfig b/configs/MPC837XERDB_defconfig
index 7054333..e6289fe 100644
--- a/configs/MPC837XERDB_defconfig
+++ b/configs/MPC837XERDB_defconfig
@@ -147,6 +147,7 @@
 CONFIG_SPCR_TSECEP_3=y
 CONFIG_LCRR_DBYP_PLL_BYPASSED=y
 CONFIG_LCRR_CLKDIV_8=y
+CONFIG_FSL_SERDES=y
 CONFIG_SYS_MONITOR_LEN=524288
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y