What is a "P2L Bridge" mentioned in Intel documentation for the
82801CAM LPC I/O controller?

Wow.  Good question.  I read through the documentation about 4 times
before I caught the important clue.

P2L stands for PCI to LPC bridge.  LPC stands for Low Pin Count and is
an interface to legacy I/O devices.  Its purpose is to simplify
motherboard and chip package layouts by minimising the number of pins
needed.

Intel's LPC interface specification (and overview) is available at
http://www.intel.com/design/chipsets/industry/lpc.htm.

It is outside the scope of this question to describe why there are so
many PCI to something bridges (including PCI to PCI bridges!), but the
simple answer is that since PCI is a fairly good standard, a lot of
things get attached to it.  In fact, virtually all low-bandwidth
devices (pretty much everything other than integrated IDE controllers
and gigE) are in one way or another chained off the PCI bus.

-Haversian