mt7622: switch default CPU governor to ondemand

mt7622: switch default CPU governor to ondemand

The most common CPU governor in the OpenWRT project is currently ondemand (see
below).  Switch mt7622 over to it as well.

Audit the code by running the following and then analyzing the results:
find -name 'config-5.*' -print0 | xargs -0 grep 'CONFIG_CPU_FREQ_DEFAULT_GOV.*=y'

ondemand: 16
performance: 5
schedutil: 5
userspace: 2

Build system: x86_64
Build-tested: ramips/mt7621

Signed-off-by: John Audia <graysky@archlinux.us>