thermal: intel: BXT_PMIC: select REGMAP instead of depending on it

[ Upstream commit 1467fb960349dfa5e300658f1a409dde2cfb0c51 ]

REGMAP is a hidden (not user visible) symbol. Users cannot set it
directly thru "make *config", so drivers should select it instead of
depending on it if they need it.

Consistently using "select" or "depends on" can also help reduce
Kconfig circular dependency issues.

Therefore, change the use of "depends on REGMAP" to "select REGMAP".

Fixes: b474303f ("thermal: add Intel BXT WhiskeyCove PMIC thermal driver")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>