ARM: mv78xx0: mark mv78xx0_timer_init as __init_refok

ARM: mv78xx0: mark mv78xx0_timer_init as __init_refok

The sys_timer init function is only called at __init time,
so it's safe to mark mv78xx0_timer_init as __init_refok,
which allows us to call orion_time_init without getting
a link time warning.

Without this patch, building mv78xx0_defconfig results in:

WARNING: vmlinux.o(.text+0x15470): Section mismatch in reference from the function mv78xx0_timer_init() to the function .init.text:orion_time_init()
The function mv78xx0_timer_init() references
the function __init orion_time_init().
This is often because mv78xx0_timer_init lacks a __init
annotation or the annotation of orion_time_init is wrong.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Andrew Lunn <andrew@lunn.ch>