ALSA: scarlett2: Add support for device map retrieval

Add support for retrieving the device map from Focusrite Scarlett 4th
Gen and Vocaster devices. The device map is a base64-encoded,
zlib-compressed JSON description of the device's capabilities and
configuration.

This patch adds:
- a has_devmap field to the scarlett2_device_info struct
- a /proc/asound/cardX/device-map.json.zz.b64 file when supported

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/e86380c6792460d8d05a8ecc37c9ebd072be25a5.1727971672.git.g@b4.vu