ARM: dts: stm32: Add SAI2A pins B pinmux entry on stm32mp1

ARM: dts: stm32: Add SAI2A pins B pinmux entry on stm32mp1

Add pinmux entry for SAI2A with alternative pin configuration.
This is useful in combination with sai2b_pins_b e.g. for codecs
like SGTL5000.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: linux-stm32@st-md-mailman.stormreply.com
To: linux-arm-kernel@lists.infradead.org
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>