kbuild: SPL/TPL: generate separate asm-offsets.h for SPL and TPL

kbuild: SPL/TPL: generate separate asm-offsets.h for SPL and TPL

Currently generic-asm-offsets.h and asm-offsets.h are generated based
on U-Boot proper config options. The same asm-offsets headers are used
for building U-Boot SPL/TPL, which causes potential offset mismatch if
U-Boot proper has different config options from U-Boot SPL/TPL.

This commit adds:
  spl/include/generated/(generic-)asm-offsets.h
  tpl/include/generated/(generic-)asm-offsets.h

spl/include/generated/(generic-)asm-offsets.h is generated if
CONFIG_SPL=y, and included when building SPL.

tpl/include/generated/(generic-)asm-offsets.h is generated if
CONFIG_TPL=y, and included when building TPL.

They are created before Kbuild descends into SPL/TPL object directories
and builds $(obj)/dts/dt-platdata.o because $(obj)/dts/dt-platdata.c
includes a bunch of headers.

Prepend -I$(obj)/include to $(UBOOTINCLUDE) so (generic-)asm-offsets.h
is searched in {spl,tpl}/include/generated/.

Requested-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Bin Meng <bmeng.cn@gmail.com>