cpufreq: dt-platdev: drop socionext,uniphier-ld6b from whitelist

As you see arch/arm/boot/dts/uniphier-ld6b.dtsi, it includes
uniphier-pxs2.dtsi, which uses "operating-points-v2" property
and whose cpufreq device is automatically created.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>