serial: Unconditionally enable CONFIG_SERIAL_MULTI

Enable CONFIG_SERIAL_MULTI for all builds of U-Boot. That includes
both SPL builds and non-SPL builds, everything. To avoid poluting
this patch with removal of ifdef-endif constructions containing
CONFIG_SERIAL_MULTI, the CONFIG_SERIAL_MULTI is temporarily added
into CPPFLAGS in config.mk . This will be again removed in following
patch.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>