soc: qcom: ramp_controller: Include linux/bitfield.h

Building ramp_controller under x86_64 results in the following build
error:

error: implicit declaration of function 'FIELD_PREP' is invalid in C99

Include linux/bitfield.h to ensure FIELD_PREP() is declared.
Signed-off-by: Bjorn Andersson <andersson@kernel.org>

drivers/soc/qcom/ramp_controller.c

@@ -5,6 +5,7 @@
  *                     <angelogioacchino.delregno@collabora.com>
  */
 
+#include <linux/bitfield.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/of.h>