phy: ti: am654-serdes: Constify regmap_config

phy: ti: am654-serdes: Constify regmap_config

regmap_config is not modified and can be made static to allow the compiler
to put it in read-only memory.

Before:
   text    data     bss     dec     hex filename
  12328    3644      64   16036    3ea4 drivers/phy/ti/phy-am654-serdes.o

After:
   text    data     bss     dec     hex filename
  12648    3324      64   16036    3ea4 drivers/phy/ti/phy-am654-serdes.o

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Link: https://lore.kernel.org/r/20200524095516.25227-2-rikard.falkeborn@gmail.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>