crypto: sa2ul - Remove child devices in remove

The sa_ul_probe creates child devices using of_platform_populate(),
but these are not cleaned up in driver remove. Clean these up
by removing the child devices using of_platform_depopulate().

Signed-off-by: Suman Anna <s-anna@ti.com>
Reviewed-by: Tero Kristo <kristo@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>