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>

dts/bindings/mtd/st,stm32f4-nv-flash.yaml

@@ -0,0 +1,20 @@
+description: |
+  ST STM32F4 family flash memory.
+
+include: st,stm32-nv-flash.yaml
+
+compatible: st,stm32f4-nv-flash
+
+properties:
+  write-block-size:
+    required: true
+    type: int
+    enum:
+      - 1
+      - 2
+      - 4
+      - 8
+    default: 1
+    description: |
+      Number of bytes used in write operations. Default value is based on the
+      reset value of Flash Control Register (FLASH_CR).

dts/bindings/mtd/st,stm32l0-nv-flash.yaml

@@ -0,0 +1,17 @@
+description: |
+  ST STM32L0 family flash memory.
+
+include: st,stm32-nv-flash.yaml
+
+compatible: st,stm32l0-nv-flash
+
+properties:
+  write-block-size:
+    required: true
+    type: int
+    enum:
+      - 1
+      - 2
+      - 4
+    description: |
+      Number of bytes used in write operations.