ARM: dts: keystone: Fix missing fallback and case in SPI NOR node compatible

Vendor prefix in compatible should be lower-case and the jedec,spi-nor
should be used as fallback, according to the bindings.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Nishanth Menon <nm@ti.com>
Link: https://lore.kernel.org/r/20220420090247.73410-2-krzysztof.kozlowski@linaro.org