Add Arm Teensys to ARM_PROCESSORS list

[fauxpark]

API error log is listing a handful of these type of messages:

Warning: ergodox_infinity: Unknown MCU: MK20DX256

[noroadsleft]

Looks good to me.

[tzarc]

Should we look at a longer-term fix using qmk/qmk_firmware#8256 ?

They get transformed during build, based on https://github.com/qmk/qmk_firmware/blob/master/quantum/mcu_selection.mk

[fauxpark]

I would like to figure out how to retain the original MCU name here, as cortex-m0 isn't all that descriptive.

[skullydazed]

Should we look at a longer-term fix using qmk/qmk_firmware#8256 ?

I'm with @fauxpark here, it'd be good to retain the MCU. If we want to extrapolate to generic names we should use a variable other than MCU.