brcmfmac: add debugfs entry for reading firmware capabilities

This allows reading all capabilities as reported by a firmware. They are
printed using native (raw) names, just like developers like it the most.
It's how firmware reports support for various features, e.g. supported
modes, supported standards, power saving details, max BSS-es.

Access to all that info is useful for trying new firmwares, comparing
them and debugging features AKA bugs.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>