rbcfg: Implement CPU frequency control

rbcfg: Implement CPU frequency control

This patch implements CPU frequency control as found on several
routerboard devices.

Supported SoCs:
- QCA953X
- AR9344

Tested on hAP lite and mAP lite (QCA953x): steps of 50MHz
Tested on LHG 5 (AR9344): steps of 50MHz

On unsupported hardware, this patch is a NOP: it will not alter the
new field.
"rbcfg help" will display an empty "cpu_freq" help listing.
"rbcfg show" will not show the cpu_freq field.
"rbcfg set/get cpu_freq" will return an error code.

Signed-off-by: Thibaut VARENE <hacks@slashdirt.org>
[adjusted subject]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>