spi: Convert the Aspeed SMC controllers device tree binding

The "interrupt" property is optional because it is only necessary for
controllers supporting DMAs (Not implemented yet in the new driver).

Cc: Chin-Ting Kuo <chin-ting_kuo@aspeedtech.com>
Tested-by: Joel Stanley <joel@jms.id.au>
Tested-by: Tao Ren <rentao.bupt@gmail.com>
Tested-by: Jae Hyun Yoo <quic_jaehyoo@quicinc.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Link: https://lore.kernel.org/r/20220509175616.1089346-3-clg@kaod.org
Signed-off-by: Mark Brown <broonie@kernel.org>