mfd: tps65910: Remove unused data

The 'io_mutex' is not used anywhere.
The regmap API supports the mutex internally, so no additional mutex
is required.

And 'domain' private data is unnecessary because the irq domain is
already registered by using regmap_add_irq_chip().

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>