ARM: serial: Remove the IXP UART driver

This driver is no longer used, remove it.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Michael Schwingen <michael@schwingen.org>
Cc: Tom Rini <trini@ti.com>