ARM: kill Hynix h720x platform

The platform was merged about 10 years ago, and has seen few updates
for most of the time since. The people that merged the code seem
no longer interested in it either, so let's remove it now.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Robert Schwebel <r.schwebel@pengutronix.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>