board: sl28: fix DRAM pretty print

The current console output is:

DRAM:  4 GiB
DDR    4 GiB (DDR3, 32-bit, CL=11, ECC on)

The size is printed twice and we can save one line of console output if
we join both lines. The new output is as follows:

DRAM:  4 GiB (DDR3, 32-bit, CL=11, ECC on)

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>