ARM: mvebu: convert secondary CPU clock sync to hotplug state

The current call site in boot_secondary is causing sleep in invalid context
warnings, as this part of the code is running with interrrupts disabled and
some of the calls into the clock framework might sleep on a mutex.

Convert the secondary CPU clock sync to a hotplug state, which allows to
call it from a sleepable context.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>