generic: routerboot sysfs: soft_config support for ath79 cpufreq

This commit introduces support for R/W access to the CPU frequency
setting of routerboot on ath79 hardware.

On unsupported hardware, the sysfs attribute will expose the raw tag
value (read-only) to help with reverse engineering its meaning.

Tested-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
Tested-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>
Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>