pxa uart driver bug

[Tsai, Tung-Chieh]

Dear all,

The uart driver of pxa in l4/pkg/arm_drivers and l4/pkg/drivers seems
forget to add volatile on Uart_pxa::wr and  Uart_pxa::rd. This still work
on qemu since qemu didn't emulate uart timing, but fail on  standard
16550 uart on real board.

And I'm confuse on l4/pkg/arm_drivers and l4/pkg/drivers. The uart directory
in them are the same, and they all build libarm_uart.a. But while building,
it'll first build drivers, then bootstrap, and then arm_drivers, and
will override
libarm_uart.a.  It seems l4/pkg/drivers are redundant.

Best Regards,
Tsai, Tung-Chieh