Allow RT3350 CPU clock to be detected as part of RT3050/RT3052 detection

OpenWRT's dts files treat RT3050/RT3052/RT3350 within the same SoC dtsi
file, so we need to distinguish between the three dynamically, mainly
because the bit we use to determine the clock speed on RT3050/RT3052
can actually be floating on RT3350 and RT3350 is always at 320MHz.

Approved by: adrian (mentor)
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D5983