Documentation/kernel-parameters: add missing pciserial to the earlyprintk

The PCI based UART can be specified for earlyprintk with the 'pciserial'
parameter from the ea9e9d80. This patch adds missing information about
this parameter.

Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>