sunxi: sun4i: improve cpu clock selection method

sunxi: sun4i: improve cpu clock selection method

clock_set_pll1 would pick the next highest available cpu clock speed if
a value not in the pre defined table was selected. this potentially
results in overclocking the soc.

reverse the selection method so that we select the next lowest speed
and add the missing 912Mhz setting that's requested by sun7i which also
uses the sun4i clock code.

Signed-off-by: Iain Paton <ipaton0@gmail.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>