dt-bindings: pinctrl: stm32: add new entry for package information

Add "st,package" entry. Possibles values are:
-STM32MP_PKG_AA for LFBGA448 (18*18) package
-STM32MP_PKG_AB for LFBGA354 (16*16) package
-STM32MP_PKG_AC for TFBGA361 (12*12) package
-STM32MP_PKG_AD for TFBGA257 (10*10) package

Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>