dmaengine: ste_dma40: fix unneeded variable warning

clang-9 points out that there are two variables that depending on the
configuration may only be used in an ARRAY_SIZE() expression but not
referenced:

drivers/dma/ste_dma40.c:145:12: error: variable 'd40_backup_regs' is not needed and will not be emitted [-Werror,-Wunneeded-internal-declaration]
static u32 d40_backup_regs[] = {
^
drivers/dma/ste_dma40.c:214:12: error: variable 'd40_backup_regs_chan' is not needed and will not be emitted [-Werror,-Wunneeded-internal-declaration]
static u32 d40_backup_regs_chan[] = {

Mark these __maybe_unused to shut up the warning.

Signed-off-by: Arnd Bergmann <[e-mail]>
Reviewed-by: Nathan Chancellor <[e-mail]>
Reviewed-by: Linus Walleij <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Vinod Koul <[e-mail]>

drivers/dma/ste_dma40.c

···
  * when the DMA hw is powered off.
  * TODO: Add save/restore of D40_DREG_GCC on dma40 v3 or later, if that works.
  */
-static u32 d40_backup_regs[] = {
+static __maybe_unused u32 d40_backup_regs[] = {
 	D40_DREG_LCPA,
 	D40_DREG_LCLA,
 	D40_DREG_PRMSE,
···
 
 #define BACKUP_REGS_SZ_V4B ARRAY_SIZE(d40_backup_regs_v4b)
 
-static u32 d40_backup_regs_chan[] = {
+static __maybe_unused u32 d40_backup_regs_chan[] = {
 	D40_CHAN_REG_SSCFG,
 	D40_CHAN_REG_SSELT,
 	D40_CHAN_REG_SSPTR,