serial: ns16550: Link in the DM driver when when using platdata

serial: ns16550: Link in the DM driver when when using platdata

Do not condition the compilation of the U_BOOT_DRIVER by !OF_PLATDATA.
This is inconsistent with the majority of other drivers. This also
blocks OF_PLATDATA boards with an 16550-compatible serial from using
serial in SPL.

Signed-off-by: Alexandru Gagniuc <alex.g@adaptrum.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Added tweak for rock to avoid a TPL build failure:
Signed-off-by: Simon Glass <sjg@chromium.org>