board: ti: am335x: Fix DM_TPS65910 condition

board: ti: am335x: Fix DM_TPS65910 condition

scale_vcores_generic() calls functions implemented in
tps65910.c, not tps65910_dm.c. Change guard from CONFIG_DM_PMIC_TPS65910 to
CONFIG_SPL_POWER_TPS65910.

Fixes: 0b9ff0851592 ("board: ti: am335x: Do not call disabled PMIC functions")
Signed-off-by: Markus Schneider-Pargmann (TI.com) <msp@baylibre.com>
Signed-off-by: Maarten Brock <maarten.brock@sttls.nl>
Reviewed-by: Kory Maincent <kory.maincent@bootlin.com>
Acked-by: Maarten Brock <maarten.brock@sttls.nl>