ARM: 7156/1: l2x0: fix compile error on !CONFIG_USE_OF

fae2b89a (ARM: l2x0: add empty l2x0_of_init) adds a static inline
function that returns -ENODEV, but at least on tegra cache-l2x0.h is
included without errno.h being pulled in first, resulting in compile
errors if OF isn't enabled:

In file included from arch/arm/mach-tegra/common.c:26:
arch/arm/include/asm/hardware/cache-l2x0.h: In function 'l2x0_of_init':
arch/arm/include/asm/hardware/cache-l2x0.h:110: error: 'ENODEV' undeclared (first use in this function)
arch/arm/include/asm/hardware/cache-l2x0.h:110: error: (Each undeclared identifier is reported only once
arch/arm/include/asm/hardware/cache-l2x0.h:110: error: for each function it appears in.)

Add errno.h to the include file to make it self-contained.

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>