ARM: OMAP: build mach-omap code only if needed

If we build a kernel with CONFIG_ARCH_OMAP2PLUS enabled but all of the
individual SoCs disabled, we run into a large number of link errors
because if incorrect dependencies:

arch/arm/mach-omap2/built-in.o: In function `_add_initiator_dep':
arch/arm/mach-omap2/omap_hwmod.c:691: undefined reference to `clkdm_add_sleepdep' arch/arm/mach-omap2/built-in.o: In function `_del_initiator_dep':
arch/arm/mach-omap2/omap_hwmod.c:720: undefined reference to `clkdm_del_sleepdep' arch/arm/mach-omap2/built-in.o: In function `_enable':
arch/arm/mach-omap2/omap_hwmod.c:2145: undefined reference to `clkdm_in_hwsup'
arch/arm/mach-omap2/omap_hwmod.c:2147: undefined reference to `clkdm_hwmod_enable'
arch/arm/mach-omap2/omap_hwmod.c:2191: undefined reference to `clkdm_hwmod_disable'
arch/arm/mach-omap2/omap_hwmod.c:2146: undefined reference to `clkdm_missing_idle_reporting' arch/arm/mach-omap2/built-in.o: In function `_idle':
arch/arm/mach-omap2/omap_hwmod.c:2235: undefined reference to `clkdm_hwmod_disable' arch/arm/mach-omap2/built-in.o: In function `_shutdown':
arch/arm/mach-omap2/omap_hwmod.c:2338: undefined reference to `clkdm_hwmod_disable' arch/arm/mach-omap2/built-in.o: In function `omap_hwmod_get_context_loss_count':
arch/arm/mach-omap2/omap_hwmod.c:4071: undefined reference to `pwrdm_get_context_loss_count' arch/arm/mach-omap2/built-in.o: In function `omap_pm_clkdms_setup':
arch/arm/mach-omap2/pm.c:114: undefined reference to `clkdm_allow_idle'
arch/arm/mach-omap2/pm.c:117: undefined reference to `clkdm_sleep' arch/arm/mach-omap2/built-in.o: In function `omap2_common_pm_late_init':
arch/arm/mach-omap2/pm.c:294: undefined reference to `omap_voltage_late_init' arch/arm/mach-omap2/built-in.o: In function `omap2_gpio_dev_init':
arch/arm/mach-omap2/gpio.c:133: undefined reference to `pwrdm_can_ever_lose_context'

We can avoid this if we make CONFIG_ARCH_OMAP2PLUS a silent option that
gets enabled any time that one of the SoC versions is enabled.

Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>