flash/nor/efm32: Support EZR32HG devices.

Recognise the family number for Silicon Labs EZR32HG devices and
select the correct flash page size.

Change-Id: I876e930f3a9f679557fa0d0acac33e9bbfb28c46
Signed-off-by: Richard Watts <rrw@kynesim.co.uk>
Reviewed-on: http://openocd.zylin.com/3934


Tested-by: jenkins
Reviewed-by: Fredrik Hederstierna <fredrik@hederstierna.com>
Reviewed-by: Jonas Norling <jonas.norling@cyanconnode.com>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>