[uefi] enable 32bit uefi support

we already have CONFIG_EFI_MIXED=y so kernel part should be fine
build passed. not runtime tested (no hardwware)

thanks @klojum and @fritsch for pointing out that such broken (and argh released in late 2014) hardware exists.

ref http://liliputing.com/2014/10/run-ubuntu-zotac-zbox-pico-mini-pc-kinda.html