ARM: stm32: Add boot counter to DHSOM

ARM: stm32: Add boot counter to DHSOM

Add boot counter to STM32MP15xx DHSOM. This aligns the software with
other upstream DHSOM products which already do enable boot counter.

The boot counter on STM32MP15xx is placed in the TAMP block TAMP_BKPxR
register 19, right past register 17 and 18 used for CM4 resource table
and state by the Linux kernel. The TAMP_BKPxR register block is used
because its contents survives warm reset, but not cold reset.

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>