[SERIAL] Allow 8250 PCI, PNP, GSC and HP300 support to be disabled

Allow the 8250 probe modules to be disabled if we're building for
with EMBEDDED enabled.  This reduces the kernel size by not including
unnecessary probe module support.

Original idea from Matt Mackall for PCI only, expanded to others by
rmk.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>