mfd: intel_soc_pmic_bxtwc: Rename header to follow c-file

For better understanding of relationship between headers and modules
rename:
	intel_bxtwc.h -> intel_soc_pmic_bxtwc.h

While here, remove file name from the file itself.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>