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

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

Cc: Greg KH <gregkh@linuxfoundation.org>
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


Cc: <stable@vger.kernel.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>