ARM: dts: at91: fix typo in sama5d2 PIN_PD24 description

Fix a typo on PIN_PD24 for UTXD2 and FLEXCOM4_IO3 which were
wrongly linked to PIN_PD23).

Signed-off-by: Florian Vallee <fvallee@eukrea.fr>
Fixes: 7f16cb67 ("ARM: at91/dt: add sama5d2 pinmux")
Cc: stable@vger.kernel.org # v4.4+
[nicolas.ferre@atmel.com: add commit message, changed subject]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>