Revert "ARCv2: Allow enabling PAE40 w/o HIGHMEM"

Revert "ARCv2: Allow enabling PAE40 w/o HIGHMEM"

This reverts commit 32ccada1fa665499dedb504877cfa9a3db833e5d.

Now when we have a real hardware platform with PAE40 enabled
(here I mean axs103 with firmware v1.2) and 1 Gb of DDR mapped to
0x1_a000_0000-0x1_ffff_ffff we're really targeting memory above 4Gb
when PAE40 is enabled. This in its turn requires HIGHMEM to be enabled
otherwise user won't see any difference with enabling PAE in
kernel configuration as only lowmem will be used anyways.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>