pmic: tps65218: add useful functions and defines

Add the following functions:
tps65218_reg_read() for accessing redisters
tps65218_toggle_fseal() for toggling the fseal bit
tps65218_lock_fsea() for locking the fseal bit to 1

Add the following defines:
All status register bits

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>