config.mk: Remove $(PCI_CLOCK) reference

config.mk: Remove $(PCI_CLOCK) reference

The following commit introduced $(PCI_CLOCK) reference so that
we could tweak `PCI_66M' definition via an environment variable.

> commit f046ccd15c8bc9613bfd72916b761a127d36e5c6
> Author: Eran Liberty <liberty@freescale.com>
> Date:   Thu Jul 28 10:08:46 2005 -0500
>
>     * Patch by Eran Liberty
>       Add support for the Freescale MPC8349ADS board.

But I suggest a removal of it for the following reasons:

* In 2006, MPC8349ADS was merged into MPC8349EMDS port,
  and it seems that MPC8349EMDS port is PCI_66M free.

* OTOH, PCI_66M is used by MPC832XEMDS an MPC8360EMDS ports,
  but they don't need $(PCI_CLOCK) environment variable at all.
  PCI_66M is automatically configured via $(BOARD)_config names
  with the help of $(findstring _66_,$@).

* Unfortunately $(PCI_CLOCK) has been undocumented anywhere,
  so only a few people know the existence of it these days.

* Keep config.mk independent from $(BOARD) as much as possible.

Signed-off-by: Shinya Kuribayashi <skuribay@pobox.com>
Acked-by: Kim Phillips <kim.phillips@freescale.com>