kbuild: move asm-offsets.c from SoC directory to arch/$(ARCH)/lib

U-Boot has supported two kinds of asm-offsets.h.

One is generic for all architectures and its source is located at
./lib/asm-offsets.c.

The other is SoC specific and its source is under SoC directory.
The problem here is that only boards with SoC directory can use
the asm-offsets infrastructure.
Putting asm-offsets.c right under CPU directory does not work.

Now a new demand is coming. PowerPC folks want to use asm-offsets.
But no PowerPC boards have SoC directory.

It seems inconsistent that some boards add asm-offsets.c to SoC
directoreis and some to CPU directories.
It looks more reasonable to put asm-offsets.c under arch/$(ARCH)/lib.

This commit merges asm-offsets.c under SoC directories into
arch/$(ARCH)/lib/asm-offsets.c.

By the way, I doubt the necessity of some entries in asm-offsets.c.
I am leaving refactoring to the board maintainers.
Please check "TODO" in the comment blocks in
arch/{arm,nds32}/lib/asm-offsets.c.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Yuantian Tang <Yuantian.Tang@freescale.com>