ARM: at91: pm: Mark at91_pm_secure_init as __init

at91_pm_secure_init() is used inside sama5d2_pm_init(), which has
the __init notation.

Pass the __init notation to at91_pm_secure_init() as well to fix the
following section mismatch warning:

WARNING: modpost: vmlinux.o(.text.unlikely+0x2138): Section mismatch in reference from the function at91_pm_secure_init() to the (unknown reference) .init.rodata:(unknown)

Fixes: f2f5cf78 ("ARM: at91: pm: add support for sama5d2 secure suspend")
Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220622114810.1186330-1-festevam@gmail.com