ARM: do not define VMALLOC_END relative to PAGE_OFFSET

VMALLOC_END is supposed to be an absolute value, while PAGE_OFFSET may
vary depending on the selected user:kernel memory split mode through
CONFIG_VMSPLIT_*.  In fact, the goal of moving PAGE_OFFSET down is to
accommodate more directly addressed RAM by the kernel below the vmalloc
area, and having VMALLOC_END move along PAGE_OFFSET is rather against
the very reason why PAGE_OFFSET can be moved in the first place.

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>