drivers/sensor: lis2ds12: Move odr Kconfig property into dts

Move odr options from Kconfigs to Device Tree. Moreover add
in DT a power-mode option to select among 4 possible values
(PD, LP, HR, HF). The power mode cannot be currently set from
sensor APIs.

Signed-off-by: Armando Visconti <armando.visconti@st.com>