ARM: stm32: Enable UNZIP on DHSOM by default

The CMD_UNZIP provides the 'gzwrite' command, which is convenient
for writing e.g. gz-compressed images to eMMC from U-Boot.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>