drivers: lora: Get rid of counter support

The RTC/Counter implementation doesn't fit for the upcoming LoRaWAN
as most of the Counter drivers in Zephyr works with 1s granularity
which is not enough for LoRaWAN stack. So, k_timer calls are used in
place of Counter's alarm and k_uptime_get() APIs are used in place of
Counter's time keeping.

While at it, lets also fixup the broken alarm implementation.

Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>