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 <[e-mail]>
Signed-off-by: Lorenzo Pieralisi <[e-mail]>
Acked-by: Subrahmanya Lingappa <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Reviewed-by: Minghuan Lian <[e-mail]>
Reviewed-by: Subrahmanya Lingappa <[e-mail]>

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

···
 	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