efi: Move unicode to ASCII conversion to shared function.

Move the open-coded conversion to a shared function for
use by all architectures.  Change the allocation to prefer
a high address for ARM, as this is required to avoid conflicts
with reserved regions in low memory.  We don't know the specifics
of these regions until after we process the command line and
device tree.

Signed-off-by: Roy Franz <roy.franz@linaro.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>