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>