ARM: davinci: PM: fix build when da850 not compiled in

Currently, suspend/resume support is only available on da850 platforms,
and the platform PM code has dependencies on da850 functions.  However,
CONFIG_SUSPEND might be enabled even when da850 support is not, causing
build failure:

arch/arm/mach-davinci/built-in.o: In function `davinci_pm_init':
pm_domain.c:(.init.text+0x1fb8): undefined reference to `da8xx_get_mem_ctlr'
pm_domain.c:(.init.text+0x20b0): undefined reference to `da8xx_syscfg1_base'

Fix this by only building the PM core when da850 is enabled.

Reported-by: Sekhar Nori <nsekhar@ti.com>
Fixes: aa9aa1ec ("ARM: davinci: PM: rework init, remove platform device")
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>