sbc8548: allow enabling PCI via a make config option

Prior to this commit, to enable PCI, you had to go manually
edit the board config header, and if you had 33MHz PCI, you
had to manually change CONFIG_SYS_NS16550_CLK too, which was
not real user friendly,

This adds the typical PCI and clock speed make targets to the
toplevel Makefile in accordance with what is being done with
other boards (i.e. using the "-t" to mkconfig).

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>