arm: vt8500: Fix build warning in uncompress.h

This removes a build-time warning in uncompress.h due to incorrectly
cast values being passed to readb/writeb.

Removed arm/io.h and readb/writeb. Read/write directly to the serial
port with a volatile pointer.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Olof Johansson <olof@lixom.net>