spi: rp2040-gpio-bridge: add missing MD5 dependency

rp2040-gpio-bridge relies on the md5 crypto driver. This dependency
cannot be determined automatically as rp2040-gpio-bridge does not
use any of md5's symbols directly.

Declare a soft 'pre' dependency on md5 to ensure that it is included and
loaded before rp2040-gpio-bridge.

Signed-off-by: Richard Oliver <richard.oliver@raspberrypi.com>