clk: Restrict CLK_HSDK to ARC_SOC_HSDK

The HSDK PLL driver is only useful when building for an ARC HSDK
platform.
As ARC selects OF, the dependency on OF can just be replaced by a
dependency on ARC_SOC_HSDK.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200807094351.1046-1-geert+renesas@glider.be


Signed-off-by: Stephen Boyd <sboyd@kernel.org>