mpc85xx: p2020: kernel: enable CONFIG_BLK_DEV_NVME

mpc85xx: p2020: kernel: enable CONFIG_BLK_DEV_NVME

Enables use of NVMe storage devices with appropriate adapter in miniPCIe slots (including for boot)
in Turris 1.x routers and possibly NXP P2020RDB boards
(these are the only currently supported p2020 devices according to docs[^1]).

Proper detection, mountability and readability was proved to be working
on Turris 1.1, OpenWrt 21.02 with similar configuration.

Increases gzip compressed kernel size by approximately 37 KiB (from 3 703 KiB to 3 740 KiB).

Should boot from those devices be possible the driver needs to be built in.
Inclusion as a module would prevent this functionality.

CONFIG_BLK_DEV_NVME=y
Includes NVMe driver in the kernel.[^2]

CONFIG_NVME_CORE=y
Selected by CONFIG_BLK_DEV_NVME.[^3] Not necessarily needed to be enabled explicitly,
but included to match the form of similar functionality implementations
for mvebu, x86_64 and rockchip_armv8 targets.

CONFIG_NVME_MULTIPATH disabled explicitly to prevent using more space than necessary.

[^1]: https://openwrt.org/docs/techref/targets/mpc85xx
[^2]: https://cateee.net/lkddb/web-lkddb/BLK_DEV_NVME.html
[^3]: https://cateee.net/lkddb/web-lkddb/NVME_CORE.html

Signed-off-by: Šimon Bořek <simon.borek@nic.cz>