ARM: stm32: Increment boot counter in SPL on DHSOM

Increment the boot counter already in U-Boot SPL instead of incrementing
it only later in U-Boot proper. This can be used by SPL to boot either of
two U-Boot copies and improve redundancy of software on the platform, e.g.
in case of full A/B update strategy.

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