PCI: brcmstb: Add DT property to control L1SS

The BRCM PCIe block has controls to enable control of the CLKREQ#
signal by the L1SS, and to gate the refclk with the CLKREQ# input.
These controls are mutually exclusive - the upstream code sets the
latter, but some use cases require the former.

Add a Device Tree property - brcm,enable-l1ss - to switch to the
L1SS configuration.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>