xen: arm: Correct register values and comment in early init_uart.

xen: arm: Correct register values and comment in early init_uart.

Set register values and comment in early init_uart to match
documentation of PL011 UART

Reading the PL011 UART documentation on
http://infocenter.arm.com/help/topic/com.arm.doc.ddi0183f/DDI0183.pdf
in sec 3.2 shows the early initialisation of the UART on the Versatile Express
is incorrect.  This fixes it.

Signed-off-by: Lars Rasmusson <Lars.Rasmusson@sics.se>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>