lmb: do away with arch_lmb_reserve()

lmb: do away with arch_lmb_reserve()

All of the current definitions of arch_lmb_reserve() are doing the
same thing -- reserve the region of memory occupied by U-Boot,
starting from the current stack address to the ram_top. Introduce a
function lmb_reserve_uboot_region() which does this, and do away with
the arch_lmb_reserve() function.

Instead of using the current value of stack pointer for starting the
reserved region, have a fixed value, considering the stack size config
value.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>