of: alloc anywhere from memblock if range not specified

early_init_dt_alloc_reserved_memory_arch passes end as 0 to
__memblock_alloc_base, when limits are not specified. But
__memblock_alloc_base takes end value of 0 as MEMBLOCK_ALLOC_ACCESSIBLE
and limits the end to memblock.current_limit. This results in regions
never being placed in HIGHMEM area, for e.g. CMA.
Let __memblock_alloc_base allocate from anywhere in memory if limits are
not specified.

Change-Id: Ifca54781b4b2ae2ccd0a3d414a488c8888d4472f
CRs-fixed: 969426
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>