arm: am437x: cm-t43: set tps fseal bit

Set TPS65218 FSEAL bit to 1 so that RTC could be powered using on-board
3V battery. This is necessary so that time and date will survive reboots and
power offs.

Cc: Tom Rini <trini@konsulko.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Reviewed-by: Tom Rini <trini@konsulko.com>