ARM: mv78xx0: avoid unused function warning

mv78xx0 produces a harmless warning when CONFIG_CACHE_FEROCEON_L2 is
disabled:

arch/arm/mach-mv78xx0/common.c:385:19: warning: 'is_l2_writethrough' defined but not used [-Wunused-function]

This avoids the warning by changing the #ifdef to an if(IS_ENABLED())
check with the same resulting object code.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>