MIPS: ath79: Use ath79_get_sys_clk_rate to get basic clock rates

Instead of accessing the rate field of the static
clock devices directly, use the recently introduced
helper function to get the rate of the basic clocks.

The static ath79_{ahb,cpu,ddr,ref}_clk variables
will be removed by a subsequent patch. The actual
change is in preparation of that.

Also move the clock frequency printing code into
the plat_time_init function. We are getting the
cpu clock rate there already so we can save an
extra call of the helper.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5782/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>