xen/arm: Allow vpl011 to be used by DomU

xen/arm: Allow vpl011 to be used by DomU

Make vpl011 being able to be used without a userspace component in Dom0.
In that case, output is printed to the Xen serial and input is received
from the Xen serial one character at a time.

Call domain_vpl011_init during construct_domU if vpl011 is enabled.

Introduce a new ring struct with only the ring array to avoid a waste of
memory. Introduce separate read_data and write_data functions for
initial domains: vpl011_write_data_xen is very simple and just writes
to the console, while vpl011_read_data_xen is a duplicate of
vpl011_read_data. Although textually almost identical, we are forced to
duplicate the functions because the struct layout is different.

To avoid mixing the output of different domains on the console, buffer
the output chars and print line by line. Unless the domain has input
from the serial, in which case we want to print char by char for a
smooth user experience.

The size of SBSA_UART_OUT_BUF_SIZE is arbitrary, choose the same size
as VUART_BUF_SIZE used in vuart.c.

Export a function named console_input_domain() to allow others to know
which domains has input at a given time.

Signed-off-by: Stefano Stabellini <stefanos@xilinx.com>
Acked-by: Julien Grall <julien.grall@arm.com>