ARM: 8310/1: l2c: Fix prefetch settings dt parsing

Allow prefetch settings overriding by device tree, in case
l2x0_cache_size_of_parse() returns value, prefetch tuning
properties are silently ignored. E.g. arm,double-linefill* and
arm,prefetch*.
This happens for example, when "cache-size" or "cache-sets"
properties haven't been filled in l2c dt node.

Comments from Fabrice Gasnier:

 Allow device tree to override the L2C prefetch settings, even when
 l2x0_cache_size_of_parse() fails to parse the cache geometry due to (eg)
 missing "cache-size" or "cache-sets" properties.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: Tomasz Figa <tomasz.figa@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>