boards: ronoth_lodev: Enable HSI48 clock

Since the commit 2ed292e1, the HSI48
clock must be explicitly enabled in the devicetree when a peripheral
that depends on the HSI48 clock is enabled.

This commit enables the HSI48 clock for the `ronoth_lodev` board
because it enables the RNG, which requires the it.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>