xen/arm: UART addresses are not always aligned to a page.

xen/arm: UART addresses are not always aligned to a page.

When calculating the virtual address of the UART,
add the page offset of the UART to the mapped address.

Create define EARLY_UART_VIRTUAL_ADDRESS and use instead
of FIXMAP_ADDR(FIXMAP_CONSOLE)

Signed-off-by: Jon Fraser <jfraser@broadcom.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>