stm32mp1: add finished good variant in board identifier

stm32mp1: add finished good variant in board identifier

Update the board info with the new coding including the finished good
variant:

Board: MBxxxx Var<CPN>.<FG> Rev.<Rev>-<BOM>

The OTP 59 coding is:
bit [31:16] (hex) => MBxxxx
bit [15:12] (dec) => Variant CPN (1....15)
bit [11:8]  (dec) => Revision board (index with A = 1, Z = 26)
bit [7:4]   (dec) => Variant FG : finished good (NEW)
bit [3:0]   (dec) => BOM (01, .... 255)

Change-Id: I4fbc0c84596419d1bc30d166311444ece1d9123f
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Yann Gautier <yann.gautier@st.com>