mediatek: replace downstream TRNGv2 driver

mediatek: replace downstream TRNGv2 driver

Replace the downstream mtk-rng-v2 driver which was acquires random bytes
from TF-A via SMC. A new approach is needed as TF-A for MT7986 has
changed and now requires to use SMC instead of directly accessing the
TRNG via MMIO. However, we can't know whether we are on old or new TF-A,
many devices (like the BananaPi BPi-R3) allow updating TF-A BL3 in the
field, so it may be of the old or new type, and the RNG driver will have
to figure it out somehow.

This currently means that MT7986 with newer TF-A has broken/non-working
HWRNG in Linux:
root@OpenWrt:~# hexdump -C /dev/hwrng
hexdump: /dev/hwrng: I/O error

Fix this by creating a new combined driver which replaces the previous
mtk-rng-v2 driver, and is able to auto-detect which convention to use
on MT7986.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>