misc: vexpress_config: Use member .priv_auto to set the private data

In current vexpress_config_probe code, it sets the uclass private data
directly. This will cause one compilation error:
drivers/misc/vexpress_config.c:114:27: error: lvalue required as left operand of assignment
  114 |  dev_get_uclass_priv(dev) = priv;
      |                           ^

In this patch we set the uclass private data through struct member
.priv_auto, and this compilation error disappears.

Signed-off-by: Wei Chen <wei.chen@arm.com>
Signed-off-by: Qi Feng <qi.feng@arm.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>