boards: nucleo_l073rz: Set LSI as LPTIM clock source

For some reason, LSE can't be used as LPTIM clock source
on nucleo_l073rz.
As a consequence, low power operations are not functional on
this platform.
Waiting for the original issue to be fixed, set LSI as LPTIM
clock source.

Partially fixes #38930

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>