Use any preallocated highmem region setup by the bootloader.
This implementation only checks for the existance of a single
region at region_index=0.

This feature allows the bootloader to preallocate highmem
regions and pass the region locations to the kernel through
the repository.  Preallocated regions can be used to hold the
initrd or other large data.  If no region info exists, the
kernel retains the old behavior and attempts to allocate the
highmem region itself.

Based on Hector Martin's patch "Get lv1 high memory region from
devtree".

CC: Hector Martin <hector@marcansoft.com>
Signed-off-by: Andre Heider <a.heider@gmail.com>
CC: Nathan Whitehorn <nwhitehorn@freebsd.org>
Signed-off-by: Geoff Levand <geoff@infradead.org>