omnia-eeprom: depend only on subtarget

omnia-eeprom: depend only on subtarget

Now that omnia-eeprom is marked nonshared building the cortex-a9 mvebu
subtarget will fail with:
ERROR: unable to select packages:
  omnia-eeprom (no such package):
    required by: world[omnia-eeprom]

This is because omnia-eeprom depends on TARGET_mvebu_cortexa9_DEVICE_cznic_turris-omnia
which will not be satisfied in buildbots since CONFIG_TARGET_ALL_PROFILES
and CONFIG_TARGET_PER_DEVICE_ROOTFS are set in which case
CONFIG_TARGET_mvebu_cortexa9_DEVICE_cznic_turris-omnia is not set.

So, lets simply depend on the mvebu/cortex-a9 subtarget.

Fixes: 371e7bef4046 ("omnia-eeprom: Mark it nonshared")
Link: https://github.com/openwrt/openwrt/pull/17007
Signed-off-by: Robert Marko <robimarko@gmail.com>