boards: cc1352r1_launchxl: fix echo samples

Recent Zephyr changes (IEEE 802.15.4 config moved from KConfig
to DeviceTree) left the TI CC1352r1_launchxl board's net examples in
a non-working state, mainly due to the fact that CC13x2 has two
IEEE802154 interfaces available (2.4GHz and sub-GHz) and none were
enabled.

This commit enables the 2.4GHz radio inside the board's DTS and also
introduces a new devicetree overlay inside the echo* samples:
'boards/boards/cc1352-enable-subg.overlay', enabling the sub-GHz
interface.

Signed-off-by: Stancu Florin <niflostancu@gmail.com>