Enable L1SS handling on RPI4 pcib

Thanks to @kevans91 for pointing me in the right direction. FreeBSD had
the same bug as Linux (see
https://bugzilla.kernel.org/show_bug.cgi?id=217276) where the ultimate
solution was to honor the brcm,enable-l1ss FDT property.

In current versions of the dtb files this property has been added by
default.

Without this on many, many pcie addin cards the pcib will Serror when
trying to assert the clreq# pin on the pcie bus. Many cards do not have
these hooked up.

PR: 260131, 277638, 277605
Reviewed-by: emaste
Signed-off-by: HP van Braam <hp@tmm.cx>
Pull-request: https://github.com/freebsd/freebsd-src/pull/1179