ARM: OMAP2+: Limit omap_read/write usage to legacy USB drivers

Drivers should no longer use omap_read/write functions
but instead use ioremap + read/write functions.

As some USB legacy code is still shared between omap1 and
omap2420, let's limit the omap_read/write to plat/usb.h.

Note that the long term fix is to update the drivers to
use ioremap and read/write functions. That can now be
done as a separate patch series that is limited to the
USB drivers.

Also make sure the legacy omap1-keypad.c driver builds
if selected for 2420 based systems.

Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>