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.com


Signed-off-by: Mark Brown <broonie@kernel.org>