MIPS: ath79: update devicetree clock support for AR9132

MIPS: ath79: update devicetree clock support for AR9132

Current ath79 clock.c code does not read reference clock and
pll setup from devicetree. E.g. you can set any clock rate value
in board DTS but it will have no effect on the real clk calculation.

This patch fixes some AR9132 devicetree clock support defects:

  * clk initialization function ath79_clocks_init_dt_ng()
    is introduced; it actually gets pll block base register
    address and reference clock from devicetree;
  * pll register parsing code is moved to the separate
    ar724x_clk_init() function; this function
    can be called from platform code or from devicetree code.

Also mips_hpt_frequency value is set from dt, so the appropriate
clock parameter is added to the cpu@0 devicetree node.

The same approach can be used for adding AR9331 devicetree support.

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Cc: Gabor Juhos <juhosg@openwrt.org>
Cc: Alban Bedel <albeu@free.fr>
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: linux-clk@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12876/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>