serial: Remove CONFIG_SERIAL_MULTI from remaining sources

Remove the parts depending either on disabled CONFIG_SERIAL_MULTI
or ifdefs around CONFIG_SERIAL_MULTI parts since CONFIG_SERIAL_MULTI
is now enabled by default.

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>

arch/sandbox/lib/board.c

@@ -220,9 +220,7 @@ void board_init_r(gd_t *id, ulong dest_addr)
 
 	gd->flags |= GD_FLG_RELOC;	/* tell others: relocation done */
 
-#ifdef CONFIG_SERIAL_MULTI
 	serial_initialize();
-#endif
 
 #ifdef CONFIG_POST
 	post_output_backlog();