sunxi: pmic_bus: Fix Kconfig dependencies

sunxi: pmic_bus: Fix Kconfig dependencies

AXP_PMIC_BUS enables communication with a specific AXP PMIC at a
PMIC-dependent I2C/P2WI/RSB bus address. It is automatically selected
as a dependency of the PMIC driver. It should not be selectable by the
user when no PMIC is chosen.

AXP_GPIO uses the pmic_bus functions, and also depends on a specific
PMIC header to pick up register definitions.

Both of these changes have no impact on any existing configs, since
the code does not compile if the dependencies are not met.

Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>