boards: st: stm32l152c_disco: add board support

The STM32L152C-DISCOVERY is a development board based on the STM32L152RC
MCU. It is very similar to the stm32l1_disco, which was used as a
reference for the pinmux and defconfig.

stm32l152c_disco board has 256 Kbytes of flash memory, 32 Kbytes of RAM
and 8 Kbytes of data EEPROM.

Signed-off-by: Maxin John <maxin.john@gmail.com>