regulator: tps51632: add register property for regmap

All TPS51632 registers are not readable/writable and
non-volatiles.

Add property of the registers whether it is readable/writable
or volatile for regmap framework.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>