configs/nanopi_neo4: new defconfig

configs/nanopi_neo4: new defconfig

A defconfig for the nanopi_neo4 was added in
d1cd9cdf269488d6adeb0cc6d2250cf6ffe1b3ed, but then removed in
8af7b11bd8a95bc41e4de46f727f3b0afde465a4 because an ARM32 compiler was
needed to build ATF, and this was not supported back then.

Thanks to the addition of package/arm-gnu-a-toolchain/, we can now
re-introduce this defconfig.

Compared to the previous defconfig, the following changes were done:

- enable BR2_TARGET_ARM_TRUSTED_FIRMWARE_NEEDS_ARM32_TOOLCHAIN
- bump kernel to 5.4
- increase default rootfs size to 70M
- use mainline u-boot
- switch to Marek Belisko as maintainer

Signed-off-by: Marek Belisko <marek.belisko@open-nandra.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>