spi: spi-mem: Add devm_spi_mem_dirmap_{create,destroy}()

Since direct mapping descriptors usually the same lifetime as the SPI
MEM device adding devm_ variants of the spi_mem_dirmap_{create,destroy}()
should greatly simplify error/remove path of spi-mem drivers making use
of the direct mapping API.

Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>