arch/arm/src/lc823450: Add SP_DMB() into lc823450_testset.c

In lc823450, ldrex and strex are not supported. So we implemented
up_testset() with H/W Mutex. However, there was a bug in memory
access order. This change ensures correct memory access order in
up_testset() for lc823450.

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>