serial: Compile drivers/serial/serial.c by default

Compile drivers/serial/serial.c by default both into SPL and into
non-SPL builds, since CONFIG_SERIAL_MULTI is now the default state.
Also having common/serial.c in by default now, it's pointless to keep
-DCONFIG_SERIAL_MULTI in CPPFLAGS any longer, so remove it as well.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Stefan Roese <sr@denx.de>

config.mk

@@ -194,7 +194,7 @@ OBJCFLAGS += --gap-fill=0xff
 gccincdir := $(shell $(CC) -print-file-name=include)
 
 CPPFLAGS := $(DBGFLAGS) $(OPTFLAGS) $(RELFLAGS)		\
-	-D__KERNEL__ -DCONFIG_SERIAL_MULTI
+	-D__KERNEL__
 
 # Enable garbage collection of un-used sections for SPL
 ifeq ($(CONFIG_SPL_BUILD),y)