MIPS: configs: Consolidate Loongson1 defconfigs

MIPS: configs: Consolidate Loongson1 defconfigs

Unify loongson{1b,1c}_defconfig into a single loongson1_defconfig.

Enable the following options by default:
 - CONFIG_SERIAL_OF_PLATFORM
 - CONFIG_RTC_DRV_LOONGSON
 - CONFIG_LOONGSON1_APB_DMA
 - CONFIG_MTD_NAND_LOONGSON1
 - CONFIG_SND_LOONGSON1_AC97

Also disable unnecessary options.

Signed-off-by: Keguang Zhang <keguang.zhang@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>