ARM: dts: k2hk-evm: set ubifs partition size for 512M NAND

The UBIFS partition size was incorrectly defined as for 128M NAND,
but on k2hk-evm installed NAND flash size is 512M.
Hence, correct it.

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>