lib/genalloc.c: export devm_gen_pool_create() for modules

Modules can use this function for creating pool.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Cc: Laura Abbott <lauraa@codeaurora.org>
Cc: Olof Johansson <olof@lixom.net>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>