flash: EFM32 GG/LG page size detection fix

Fixed flash page size detection according to EFM32 GG/LG errata.
MEM_INFO_PAGE_SIZE register containts invalid value in devices with
revision number lower than 18 and should not be used.

Change-Id: Idb2832246efcbbec2fd98a5c458f72a36df386fb
Signed-off-by: Roman D <me@iamroman.org>
Reviewed-on: http://openocd.zylin.com/1116
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>