qualcommax: ipq807x: move subtarget specific kernel options

Currently, qualcommax target contains the full kernel config for the
ipq807x subtarget, but since I am working on ipq60xx as well it makes
sense to split out the ipq807x specific kernel options to subtarget
config.

ipq60xx will use the same approach and use subtarget config.

Should result in the same end kernel config, verified by comparing the
generated kernel .config.

Signed-off-by: Robert Marko <robimarko@gmail.com>