ppc: qemu: Drop fixed_sdram()

This function is not called anywhere. Only fsl_ddr_sdram_size() is
necessary [1] for QEMU. Drop it.

[1] arch/powerpc/cpu/mpc85xx/cpu.c::dram_init()

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>