doc, kernel-parameters: Document 'console=hvc<n>'

commit a2fd6419174470f5ae6383f5037d0ee21ed9833f upstream.

Both the PowerPC hypervisor and Xen hypervisor can utilize the
hvc driver.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Link: http://lkml.kernel.org/r/1361825650-14031-3-git-send-email-konrad.wilk@oracle.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>