ARM: exynos: Only build MCPM support if used

We get a link error for configurations that enable an Exynos
SoC that does not require MCPM, but then manually enable
MCPM anyway without also turning on the arm-cci:

arch/arm/mach-exynos/mcpm-exynos.o: In function `exynos_pm_power_up_setup':
mcpm-exynos.c:(.text+0x8): undefined reference to `cci_enable_port_for_self'

Change it back to only build the code we actually need, by
introducing a CONFIG_EXYNOS_MCPM that serves the same purpose
as the older CONFIG_EXYNOS5420_MCPM.

Fixes: 2997520c ("ARM: exynos: Set MCPM as mandatory for Exynos542x/5800 SoCs")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>