powerpc/82xx: remove spidev node from mgcoge

Commit 956b200a ("spi: spidev: Warn loudly if instantiated from DT
as "spidev"") states that there should not be spidev nodes in DTs.
Remove this non-HW description. There won't be a regression because it
won't bind since 2015 anyhow.

Fixes: 5d1d67e3 ("powerpc/82xx: add SPI support for mgcoge")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220824082130.21934-3-wsa+renesas@sang-engineering.com