powerpc/t4240qds: Update RCW, defconfig and maintainer

1. board/freescale/t4qds/t4_rcw.cfg
1.8GHz support is requested as default frequency, so update the rcw.

2. remove un-used configs
configs/T4160QDS_SPIFLASH_defconfig
configs/T4240QDS_SPIFLASH_defconfig
SPI boot is not available on T4QDS, so the configs should be removed.

3. board/freescale/t4qds/MAINTAINERS
Updated MAINTAINERS accordingly.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
[York Sun: revise subject]
Reviewed-by: York Sun <yorksun@freescale.com>