boards: bl5340_dvk: add missing sdhc0 nodelabel

bl5340_dvk_cpuapp_common.dts is missing a label for the sdhc@0 node,
making the board failing to build on main since:

a5d16f32 boards: Add sdhc0 alias for testing purposes

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>