PCI: imx6: Simplify switch-case logic by involve core_reset callback

PCI: imx6: Simplify switch-case logic by involve core_reset callback

Instead of using the switch case statement to assert/dassert the core
reset handled by this driver itself, let's introduce a new callback
core_reset() and define it for platforms that require it.

This simplifies the code.

Link: https://lore.kernel.org/linux-pci/20240729-pci2_upstream-v8-5-b68ee5ef2b4d@nxp.com
Signed-off-by: Frank Li <Frank.Li@nxp.com>
[kwilczynski: commit log]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>