flash: efm32: add support for EFR-familty (e.g. bluegecko)

flash: efm32: add support for EFR-familty (e.g. bluegecko)

This patch adds support for Blue Gecko and Mighty Gecko chips from
Silabs.

They have different EFM32_MSC_REGBASE and LOCK register offset.

Based on the original patch from Andreas Kemnade.

Change-Id: I166c14960ced7c880b68083badd1b31372fefabe
Cc: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Reviewed-on: http://openocd.zylin.com/4034
Reviewed-by: Jonas Norling <jonas.norling@cyanconnode.com>
Tested-by: jenkins
Reviewed-by: Fredrik Hederstierna <fredrik@hederstierna.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-by: chrysn <chrysn@fsfe.org>