dm: serial: ns16550: Add pre-reloc flag to the driver

For platforms that don't use device tree in SPL the only way to mark this
driver as 'required by relocation' is with the DM_FLAG_PRE_RELOC flag. Add
this to ensure that the driver is bound.

Fixes: fde7e189 ("dm: tegra: pci: Move CONFIG_PCI_TEGRA to Kconfig")
Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Thomas Chou <thomas@wytron.com.tw>