powerpc: t1024: Fix SRDS_MAX_LANES value

T1023 and T1024 have 4 SerDes lanes. Fix macro SRDS_MAX_LANES
and use this macro instead of hard-coded value in t1024_serdes.c.

Signed-off-by: Paulo Zaneti <paulo.zaneti@datacom.ind.br>
Signed-off-by: York Sun <york.sun@nxp.com>