ARM: dts: r8a7743: Add OPP table for frequency scaling

Add needed information inside CPU0 for the generic cpufreq-cpu0 driver.

- clock-latency = 300 us
Approximate worst-case latency to do clock transition for every
OPPs. Using an arbitrary safe value similar to r8a7791(R-Car M2) Soc.

- operating-points = < kHz - uV >
List of 6 operating points. All of them are using the same voltage
since DVS is not supported in RZ/G1 Soc.

Note:This also fixes the below errors seen on kernel logs
[    0.876877] cpu cpu0: dev_pm_opp_get_opp_count: OPP table not found (-19)
[    0.883727] cpu cpu1: cpufreq_init: failed to get clk: -2

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>