serial: sh-sci: Move runtime PM enable to sci_probe_single()

serial: sh-sci: Move runtime PM enable to sci_probe_single()

Relocate the runtime PM enable operation to sci_probe_single(). This change
prepares the codebase for upcoming fixes.

While at it, replace the existing logic with a direct call to
devm_pm_runtime_enable() and remove sci_cleanup_single(). The
devm_pm_runtime_enable() function automatically handles disabling runtime
PM during driver removal.

Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Link: https://lore.kernel.org/r/20250116182249.3828577-3-claudiu.beznea.uj@bp.renesas.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>