There's only a single caller of vexpress_config_set_master() from
vexpress-sysreg.c. Let's just make the registers needed available to
vexpress-config and move all the code there. The registers needed aren't
used anywhere else either. With this, we can get rid of the private API
between these 2 drivers.

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