serial: 8250/omap1: include linux/soc/ti/omap1-soc.h

As a preparation for cleaning up the omap1 headers, start
including linux/soc/ti/omap1-soc.h directly so we can
keep calling cpu_is_omap1510().

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>