PCI: imx6: Simplify reset handling by using *_FLAG_HAS_*_RESET

Refactor the reset handling logic in the imx6 PCI driver by adding
IMX6_PCIE_FLAG_HAS_*_RESET bitmask define for drvdata::flags.

The drvdata::flags and bitmask ensure a cleaner and more scalable
switch-case structure for handling reset.

Link: https://lore.kernel.org/r/20240220161924.3871774-4-Frank.Li@nxp.com


Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>