drivers: ieee802154: dw1000: use spi_dt_spec

Simplify driver implementation by using spi_dt_spec. Note that driver
has 2 SPI configurations, identical except the speed. For this reason,
the slow config is still kept in RAM and copied from the one obtained
via the SPI dt_spec macros. A better solution would be to have macros
that allow to override the SPI frequency, but this can be improved
later. Most internal helpers have been adjusted to accept a device
reference to make SPI (and future GPIO) transition easier.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>