arm64: reset: disable cache and MMU for safety

When Zephyr boot from EL2, D-Cache, I-Cache and MMU should be disabled
by default, but in some cases, for example use U-Boot "go" command
to boot Zephyr Image, Cache and MMU are already been enabled by U-Boot.
So disable them at reset code to make it safety when the code goes
into C code.

Signed-off-by: Jiafei Pan <Jiafei.Pan@nxp.com>