clk: meson: gxbb: fix protection against undefined clks

gxbb clock driver gracefully handles case where the clkid is defined but
the clock hw pointer is not provided, as long as it is not at the end of
the hw_onecell_data array.

This patch ensure that the last entries are defined as well to handle
this particular case.

Fixes: a70c6e06

 ("clk: meson: gxbb: protect against holes in the onecell_data array")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>