flash: stm32: configurable write block sizes

Add new devicetree bindings for F4 and L1 series for configuration of
block size used in flash write operations.

Allow byte-size write operations in `flash_stm32f1x.c`. This file is
being shared between F0, F1, F3, L0 and L1 series. L0 and L1 series
allows for single byte writes.

Signed-off-by: Gustavo Silva <gustavograzs@gmail.com>