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

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 <[email protected]>
Cc: Marek Vasut <[email protected]>
Cc: Tom Rini <[email protected]>
Cc: Anatolij Gustschin <[email protected]>
Cc: Stefan Roese <[email protected]>