ARM: imx: Add i.MX7ULP SoC serial number support

i.MX7ULP's unique ID layout in OCOTP is different from other
i.MX6/7 SoCs as below:

OCOTP layout		unique ID

0x4b0 bit[15:0]		bit[15:0]
0x4c0 bit[15:0]		bit[31:16]
0x4d0 bit[15:0]		bit[47:32]
0x4e0 bit[15:0]		bit[63:48]

Add support for reading serial number from OCOTP on i.MX7ULP.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>