ath79: add env-size for Sitecom WLR-7100 / WLR-8100 u-boot-env

ath79: add env-size for Sitecom WLR-7100 / WLR-8100 u-boot-env

The Linux kernel assumes that the u-boot environment covers the full
partition, but it only covers 0x1000 bytes. Linux checks the CRC and
does this over the full partition. This fails like this:
```
u-boot-env-layout 1f000000.spi:flash@0:partitions:partition@30000:nvmem-layout: Invalid calculated CRC32: 0xfcac8c41 (expected: 0x14e6335a)
u-boot-env-layout 1f000000.spi:flash@0:partitions:partition@30000:nvmem-layout: probe with driver u-boot-env-layout failed with error -22
```

Define the u-boot environment with a length of 0x1000 bytes to calculate
the CRC only over this area.

When replicating the u-boot environment with these parameters it
generates the same CRC:
```
mkenvimage -p 0 -b -s 0x1000 -o output.bin input.txt
```

Fixes: https://github.com/openwrt/openwrt/issues/21696
Fixes: 5e3a602def72 ("ath79: sitecom,wlrx100: use nvmem")
Link: https://github.com/openwrt/openwrt/pull/22030
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>