arm64: dts: amlogic: sm1-bananapi: lower SD card speed for stability

arm64: dts: amlogic: sm1-bananapi: lower SD card speed for stability

Users report being able to boot (u-boot) from SD card but kernel
init then fails to mount partitions on the card containing boot
media resulting in first-boot failure. System logs show only the
probe of the mmc devices: the SD card is seen, but no partitions
are found so init fails to mount them and boot stalls.

Reducing the speed of the SD card from 50MHz to 35MHz results in
complete probing of the card and successful boot.

Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Link: https://lore.kernel.org/r/20250505143700.4029484-1-christianshewitt@gmail.com
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>