apm821xx: disable NVMEM_U_BOOT_ENV

apm821xx: disable NVMEM_U_BOOT_ENV

The main point of it currently is to extract mac addresses. That is not
being done as MAC addresses are elsewhere.

Disable it until it becomes more feature packed and there's an actual
use for it.

All devices already have config definitions. NVMEM prevents redundant
support as well as write support.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/16618
(cherry picked from commit 32256552362d393e66098cafaa7366f33f025144)
Link: https://github.com/openwrt/openwrt/pull/22339
Signed-off-by: Robert Marko <robimarko@gmail.com>