ARM64: juno: disable NOR flash node by default

After discussing on the mailing list it turns out that
accessing the flash memory from the kernel can disrupt CPU
sleep states and CPU hotplugging, so let's disable this
DT node by default. Setups that want to access the flash
can modify this entry to enable the flash again.

Quoting Sudeep Holla: "the firmware assumes the flash is
always in read mode while Linux leaves NOR flash in
"read id" mode after initialization."

Reported-by: Sudeep Holla <sudeep.holla@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Liviu Dudau <Liviu.Dudau@arm.com>
Cc: Lorenzo Pieralisi <Lorenzo.Pieralisi@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Leif Lindholm <leif.lindholm@arm.com>
Cc: Ryan Harkin <ryan.harkin@linaro.org>
Fixes: 5078f77e "ARM64: juno: add NOR flash to device tree"
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>