dts: arm: st: stm32h5: add backup sram to all socs

This commit moves the backup sram definition to the
series base dtsi file, the size is overwritten for socs
which have a bigger bkpsram.

The backup SRAM is available on all stm32h5 mcus.
stm32h503/523/533 have 2k
stm32h562/563/573 have 4k

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>