ramips: iptime,t5004: fix U-Boot environment size

ramips: iptime,t5004: fix U-Boot environment size

The U-Boot environment data on the ipTIME T5004 occupies one
128 KiB NAND erase block.

The nvmem layout specified an environment size of 0x1000.
This caused CRC32 validation to fail and prevented the MAC
address cells from being created.

Set env-size to 0x20000 to match the U-Boot environment data
size used for CRC validation.

Fixes: 8dd95cc6b532 ("ramips: iptime,t5004: use nvmem for ubootenv")
Signed-off-by: Myeongjae SONG <devparlair@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/24530
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>