serial: Rename driver vcth to vct to support other board variants

Moved driver vcth.c to vct.c to better reflect the VCT board series.
This driver is now used by the VCT platforms:

vct_premium
vct_platinum
vct_platinumsvc

Signed-off-by: Stefan Roese <sr@denx.de>
diff --git a/drivers/serial/Makefile b/drivers/serial/Makefile
index 17235ff..c7a1882 100644
--- a/drivers/serial/Makefile
+++ b/drivers/serial/Makefile
@@ -38,7 +38,7 @@
 COBJS-$(CONFIG_XILINX_UARTLITE) += serial_xuartlite.o
 COBJS-$(CONFIG_SCIF_CONSOLE) += serial_sh.o
 COBJS-$(CONFIG_USB_TTY) += usbtty.o
-COBJS-$(CONFIG_VCTH_SERIAL) += vcth.o
+COBJS-$(CONFIG_VCT_SERIAL) += vct.o
 
 COBJS	:= $(sort $(COBJS-y))
 SRCS	:= $(COBJS:.o=.c)