mfd: vexpress-sysreg: Drop unused syscon child devices

The "sys_id", "sys_misc" and "sys_procid" devices don't have a user
anywhere in the tree and do nothing more than create a syscon regmap for
a single register or 2. That's an overkill for creating child devices.
Let's just remove them.

Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>