MIPS: Loongson: Merge load addresses

Systems based upon the Loongson 1B & 1C CPUs share the same load
address, as do those based upon Loongson 1A. Unify the definition of
this load address to reduce duplication & avoid the need for an extra
Loongson 1A case in future.

[paul.burton@mips.com: Rewrite commit message.]

Signed-off-by: 谢致邦 (XIE Zhibang) <Yeking@Red54.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/14927/
Cc: linux-mips@linux-mips.org