This fixes a multiplatform regression on the Ux500.

When compiling the Ux500 platforms in multiplatform configurations
both PL310_ERRATA_588369 and PL310_ERRATA_727915 would crash the
platform when trying to launch the init process.

The Ux500 cannot access the debug registers of the PL310, it will
just crash if you try this. So disable this by setting the debug
callback to NULL when initializing the l2x0 on this platform.

Cc: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>