drivers: cc13xx_cc26xx: pinctrl: fix header conflict

CC13/26xx's pinctrl_cc13xx_cc26xx.c driver included ioc.h and
(indirectly) pinctrl_soc.h which contained duplicate defines.

This change removes the header conflict and redundant definitions.

This prepares for subsequent changes in this change set that add
additional flags to the pinctrl driver which would otherwise trigger the
header conflict.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>