boards: arm: Add Legend 2.5" boards

This commit adds support for the Legend 2.5" boards (legend25_ssd and
legend25_hdd) based on the STM32F070CB MCU. These boards can be found in
the Seagate FireCuda Gaming Drive, Gaming Drive for Xbox, SSD Gaming
Drive for Xbox, and Gaming Drive for PlayStation devices. Both boards
contain the following hardware components:
- A B1414 LED strip connected to the PA7 pin (SPI MOSI)
- A SPI flash (FM25F005) connected on SPI2 bus

The Legend 2.5" HDD board also contains an activity LED connected on
TIM3 CH3

Signed-off-by: Maxime Bittan <maxime.bittan@seagate.com>
Signed-off-by: Simon Guinot <simon.guinot@seagate.com>