mfd: prcmu: delete pin control helpers

These static inlines are duplicating the task now done by the
Nomadik pinctrl drivers, so delete them from the prcmu static
inlines, also delete the register definitions as these should
only be known by the pinctrl driver.

Cc: Loic Pallardy <loic.pallardy@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Cc: Michel Jaouen <michel.jaouen@st.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>