spi: Update NXP Flexspi maintainer details

Add Han Xu as flexspi maintainer.
Also, update my email address as previous one is not working anymore.
Signed-off-by: Kuldeep Singh <singh.kuldeep87k@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220302192915.6193-1-singh.kuldeep87k@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-nxp-fspi.yaml

@@ -7,7 +7,8 @@ $schema: http://devicetree.org/meta-schemas/core.yaml#
 title: NXP Flex Serial Peripheral Interface (FSPI)
 
 maintainers:
-  - Kuldeep Singh <kuldeep.singh@nxp.com>
+  - Han Xu <han.xu@nxp.com>
+  - Kuldeep Singh <singh.kuldeep87k@gmail.com>
 
 allOf:
   - $ref: "spi-controller.yaml#"