ssb: host_soc depends on sprom

Drivers that use the SSB sprom functionality typically 'select SSB_SPROM'
from Kconfig, but CONFIG_SSB_HOST_SOC misses this, which results in
a build failure unless at least one of the other drivers that selects
it is enabled:

drivers/built-in.o: In function `ssb_host_soc_get_invariants':
(.text+0x459494): undefined reference to `ssb_fill_sprom_with_fallback'

This adds the same select statement that is used elsewhere.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 541c9a84

 ("ssb: pick SoC invariants code from MIPS BCM47xx arch")
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>