lantiq: xrx200: add env-size for P-2812HNU-F1 u-boot-env

lantiq: xrx200: add env-size for P-2812HNU-F1 u-boot-env

The u-boot-env partition on the Zyxel P-2812HNU-F1 is 128 KiB but
the actual environment data is only 8 KiB (0x2000 bytes). Without
the env-size property, the u-boot,env nvmem layout driver assumes
env-size equals the full partition size and computes an incorrect
CRC32 over the entire 128 KiB, causing all nvmem cell reads to fail
silently.

Add env-size = <0x2000> so the CRC32 is computed over the correct
8 KiB region, allowing nvmem-cells (such as ethaddr) to be read
properly by the kernel.

Signed-off-by: Burak Aydos <byhexadecimal@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21780
Signed-off-by: Robert Marko <robimarko@gmail.com>