mfd: tps6594: Explicitly include bitfield.h

mfd: tps6594: Explicitly include bitfield.h

After a recent change that started using FIELD_GET() in tps6594-core.c,
there is an error when bitfield.h is not implicitly included, such as
when building allmodconfig for ARCH=hexagon:

  drivers/mfd/tps6594-core.c:767:7: error: call to undeclared function 'FIELD_GET'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    767 |                 if (FIELD_GET(TPS65224_MASK_EN_PB_VSENSE_CONFIG, pwr_on) == TPS65224_EN_SEL_PB ||
        |                     ^

Explicitly include bitfield.h to resolve the errors.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: d766ca01c208 ("mfd: tps6594: Add power button functionality")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Michael Walle <mwalle@kernel.org>
Closes: https://lore.kernel.org/oe-kbuild-all/202509032356.LGa5hygM-lkp@intel.com/
Link: https://lore.kernel.org/r/20250904-mfd-tps6594-core-fix-bitfield-h-v1-1-5d0f00cfe58f@kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>