ARM: at91: don't select CONFIG_ARM_CPU_SUSPEND for old platforms

My previous patch fixed a link error for all at91 platforms when
CONFIG_ARM_CPU_SUSPEND was not set, however this caused another
problem on a configuration that enabled CONFIG_ARCH_AT91 but none
of the individual SoCs, and that also enabled CPU_ARM720 as
the only CPU:

warning: (ARCH_AT91 && SOC_IMX23 && SOC_IMX28 && ARCH_PXA && MACH_MVEBU_V7 && SOC_IMX6 && ARCH_OMAP3 && ARCH_OMAP4 && SOC_OMAP5 && SOC_AM33XX && SOC_DRA7XX && ARCH_EXYNOS3 && ARCH_EXYNOS4 && EXYNOS5420_MCPM && EXYNOS_CPU_SUSPEND && ARCH_VEXPRESS_TC2_PM && ARM_BIG_LITTLE_CPUIDLE && ARM_HIGHBANK_CPUIDLE && QCOM_PM) selects ARM_CPU_SUSPEND which has unmet direct dependencies (ARCH_SUSPEND_POSSIBLE)
arch/arm/kernel/sleep.o: In function `cpu_resume':
(.text+0xf0): undefined reference to `cpu_arm720_suspend_size'
arch/arm/kernel/suspend.o: In function `__cpu_suspend_save':
suspend.c:(.text+0x134): undefined reference to `cpu_arm720_do_suspend'

This improves the hack some more by only selecting ARM_CPU_SUSPEND
for the part that requires it, and changing pm.c to drop the
contents of unused init functions so we no longer refer to
cpu_resume on at91 platforms that don't need it.

Fixes: cc7a938f ("ARM: at91: select CONFIG_ARM_CPU_SUSPEND")
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>