configs: sandbox64: Enable MC34708 driver

Align the sandbox64 defconfig with sandbox defconfig. Enable missing
MC34708 PMIC driver. This fixes ut_dm_dm_test_power_pmic_mc34708_get test .

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>