ARM: i.MX: system.c: Replace magic numbers

Replace magic numbers used to form L310 Prefetch Control Register value.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>