configs: sc5xx: Set default bootcmd to SPI boot

configs: sc5xx: Set default bootcmd to SPI boot

Align configs to use the same default bootcommand. With the environment
no longer stored in the SPI, SPI boot is the intended default boot
method for all sc5xx platforms, with the exception of the sc573, and the
sc584.

The sc59x boards, the sc594 and the sc598, additionally have the option
of using OSPI for boot, but the default boot method for these boards is
still SPI.

Signed-off-by: Caleb Ethridge <caleb.ethridge@analog.com>
Reviewed-by: Simon Glass <sjg@chromium.org>