The arch was renamed to stm32mp, so it doesn't look out of place when
the stm32mp2 is released. Fix spotted comments/labels with the old
name. While at it, fix a typo about the SoC name on the DK2 board.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>