ARM: vexpress: Move vexpress_flags_set() into arch code

vexpress_flags_set() is only used by the platform SMP related code and
has nothing to do with the vexpress-sysreg MFD driver other than both
access the same h/w block. It's also only needed for 32-bit systems and
must be built-in for them. Let's move vexpress_flags_set() closer to
where it is being used. This will allow for vexpress-sysreg to be built
as a module.

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