These changes were obtained by running a script  created by
Ulf Magnusson <Ulf.Magnusson@nordicsemi.no> for the following
specification:

1. Read the contents of all dts_fixup.h files in Zephyr
2. Check the left-hand side of the #define macros (i.e. the X in
   #define X Y)
3. Check if that name is also the name of a Kconfig option
   3.a If it is, then do nothing
   3.b If it is not, then replace CONFIG_ with DT_ or add DT_ if it
       has neither of these two prefixes
4. Replace the use of the changed #define in the code itself
   (.c, .h, .ld)

Additionally, some tweaks had to be added to this script to catch some
of the macros used in the code in a parameterized form, e.g.:
- CONFIG_GPIO_STM32_GPIO##__SUFFIX##_BASE_ADDRESS
- CONFIG_UART_##idx##_TX_PIN
- I2C_SBCON_##_num##_BASE_ADDR
and to prevent adding DT_ prefix to the following symbols:
- FLASH_START
- FLASH_SIZE
- SRAM_START
- SRAM_SIZE
- _ROM_ADDR
- _ROM_SIZE
- _RAM_ADDR
- _RAM_SIZE
which are surprisingly also defined in some dts_fixup.h files.

Finally, some manual corrections had to be done as well:
- name##_IRQ -> DT_##name##_IRQ in uart_stm32.c

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>