arm64: dts: ls1012a: Add serial alias for ls1012a-rdb

arm64: dts: ls1012a: Add serial alias for ls1012a-rdb

U-boot atempts to read serial alias value for ls1012a-rdb but couldn't
do so as it is not initialised and thus, FDT_ERR_NOTFOUND error is
reported while booting linux.

Loading fdt from FIT Image at a0000000 ...
   Description:  ls1012ardb-dtb
     Type:         Flat Device Tree
     Data Start:   0xab111474
     Data Size:    11285 Bytes = 11 KiB
     Architecture: AArch64
     Load Address: 0x90000000
   Loading fdt from 0xab111474 to 0x90000000
   Booting using the fdt blob at 0x90000000
   Uncompressing Kernel Image
   Loading Device Tree to 000000008fffa000, end 000000008ffffc14 ... OK
WARNING: fdt_fixup_stdout: could not read serial0 alias: FDT_ERR_NOTFOUND
NOTICE:  RNG: INSTANTIATED

Starting kernel ...

Fix the above error by specifying serial value to duart.

Signed-off-by: Kuldeep Singh <kuldeep.singh@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>