ARM: 8510/1: rework ARM_CPU_SUSPEND dependencies

The code enabled by the ARM_CPU_SUSPEND config option is used by
kernel subsystems for purposes that go beyond system suspend so its
config entry should be augmented to take more default options into
account and avoid forcing its selection to prevent dependencies
override.

To achieve this goal, this patch reworks the ARM_CPU_SUSPEND config
entry and updates its default config value (by adding the BL_SWITCHER
option to it) and its dependencies (ARCH_SUSPEND_POSSIBLE), so that the
symbol is still selected by default by the subsystems requiring it and
at the same time enforcing the dependencies correctly.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>