mediatek: enable support for vfpv4 and neon

While researching for the armvirt target, I looked at the
existing arm platforms. It turns out that the mediatek target
with its sole MT7623N/A chip is sold as a "highly integrated
multimedia network router system-on-chip". To that end, it
lists support for the "NEON multimedia processing engine with
SIMDv2 / VFPv4 ISA support".

<http://topics.mediatek.com/en/products/connectivity/wifi/home-network/wifi-ap/mt7623na/>

So this patch enables the CPU_SUBTYPE to use this information.
This should have the nice side effect that LEDE's phase2 builders
no longer need to built a separate "cortex-a7" target, so this
should free up some resources.

Cc: John Crispin <john@phrozen.org>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>