da850evm: pass board revision info to kernel

there are two boards based on da850 SOC - OMAP-L138 and AM18x.
In order to differentiate between these two boards, revision id
is passed to kernel via second byte of ATAG_REVISION.

Signed-off-by: Manjunathappa, Prakash <prakash.pm@ti.com>
Signed-off-by: Manjunath Hadli <manjunath.hadli@ti.com>