dt-bindings: PCI: mobiveil: Change gpio_slave and apb_csr to optional

Change the "gpio_slave" and "apb_csr" to optional, the "gpio_slave"
is not used in current code, and "apb_csr" is not used by some platforms.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Subrahmanya Lingappa <l.subrahmanya@mobiveil.co.in>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Minghuan Lian <Minghuan.Lian@nxp.com>
Reviewed-by: Subrahmanya Lingappa <l.subrahmanya@mobiveil.co.in>