clk: meson: meson8b: fix protection against undefined clks

meson8b 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: e92f7cca ("clk: meson8b: clean up fixed rate clocks")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>