ARM: ux500: Enable HIGHMEM in the u8500 defconfig

In order to utilise all of the memory located on ux500 based devices
we have to enable HIGHMEM. Without it the kernel truncates memory down
to what's left after the PAGE_OFFSET has been applied, which doesn't
leave an awful lot, especially if we're running large rootfs' such
as full Linux desktop distributions or Android.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>