Enhance Armada 38x SoC identification string

[mw_semihalf.com]

Summary

Add hw_clockrate and CPU frequency, basing on sample-at-reset
configuration.

[andrew]

I would like it if rather than using __weak_reference we were to architect the code properly.

sys/arm/mv/mv_common.c

424

This won't work in a kernel that supports multiple implementations of get_cpu_freq.

[mw_semihalf.com]

Instead of using weak_reference for get_cpu_clk, provide dummy functions for all other Marvell SoC's (same way as get_tclk is done).