regulator: tps65090: Use generic regmap enable/disable operations

This patch converts tps65090 regulator driver to use generic regmap
enable/disable operations.

Also move struct tps65090 to include/linux/mfd/tps65090.h because
the regulator driver needs to access the rmap field of struct tps65090.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Venu Byravarasu <vbyravarasu@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>