MIPS: Decrease size of au1xxx_dbdma_pm_regs[][]

There are 16 individual channels (NUM_DBDMA_CHANS) to save/restore plus the
global ddma block config (the +1).  The last register in a channel can be
skipped since it's read-only (at offset 0x18).

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Cc: Manuel Lauss <manuel.lauss@googlemail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>