xen/arm: Don't use pl011 UART by default for early printk

xen/arm: Don't use pl011 UART by default for early printk

Add CONFIG_EARLY_PRINTK options in configs/arm{32,64}.mk to let the user
to choose if he wants to have early output, ie before the console is initialized.

This code is specific for each UART. When CONFIG_EARLY_PRINTK is enabled,
Xen will only be able to run on a board with this UART.

If a developper wants to add support for a new UART, he must implement the
following assembly macro/define:
   - EALY_UART_BASE_ADDRESS: variable which contains the physical base address
   for the UART
   - early_uart_init: initialize the UART
   - early_uart_ready: check and wait until the UART can transmit a new
   character
   - early_uart_transmit: transmit a character

For more details about the parameters of each function,
see arm{32,64}/debug-pl011.inc comments.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>