boards/arm: Add initial support for SiLabs Giant Gecko GG11 STK

This commit adds initial support for the Silicon Labs EFM32
Giant Gecko GG11 StarterKit.

Features supported for now are NVIC, SysTick, GPIO, Flash,
Counter, I2C, UART and Ethernet. Support for Watchdog and
ADC will follow as soon as their respective PRs are merged.

Signed-off-by: Oane Kingma <o.kingma@interay.com>