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: NHou Zhiqiang <Zhiqiang.Hou@nxp.com>
Signed-off-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: NSubrahmanya Lingappa <l.subrahmanya@mobiveil.co.in>
Acked-by: NRob Herring <robh@kernel.org>
Reviewed-by: NMinghuan Lian <Minghuan.Lian@nxp.com>
Reviewed-by: NSubrahmanya Lingappa <l.subrahmanya@mobiveil.co.in>

Documentation/devicetree/bindings/pci/mobiveil-pcie.txt

@@ -10,8 +10,10 @@ Required properties:
 	interrupt source. The value must be 1.
 - compatible: Should contain "mbvl,gpex40-pcie"
 - reg: Should contain PCIe registers location and length
+	Mandatory:
 	"config_axi_slave": PCIe controller registers
 	"csr_axi_slave"	  : Bridge config registers
+	Optional:
 	"gpio_slave"	  : GPIO registers to control slot power
 	"apb_csr"	  : MSI registers