regulator: tps65910: Remvoe tps65910_reg_[read|modify_bits|read_locked|write_locked] functions

The tps65910 mfd driver has been converted to regmap APIs.
This patch adds tps65910_reg_update_bits() in include/linux/mfd/tps65910.h.
Thus we can use tps65910_reg_read/tps65910_reg_write/tps65910_reg_update_bits
directly and remove tps65910_reg_[read|modify_bits|read_locked|write_locked]
functions. With this change, we can also remove the mutex in struct tps65910_reg.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Tested-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>