powerpc: mpc5200: Correct return value of memcpy function

The memcpy() function returns a pointer to trg.

Signed-off-by: Mark Marshall <Mark.Marshall@omicron.at>
Reviewed-by: Thomas Graziadei <thomas.graziadei@omicronenergy.com>
Reviewed-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: York Sun <york.sun@nxp.com>