stm32f1x: use register base instead of register offset

stm32f1x: use register base instead of register offset

Access the different flash banks' registers using a bank specific register
base and a register specific offset. This is equivalent but feels more
natural.

Some accesses were discovered that maybe should not be hard coded to bank0
registers. Add a note about that.

Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>