Nokia RX-51: Fix compilation with non-zero CONFIG_SYS_TEXT_BASE

For some unknown reason GNU assembler version 2.31.1 (arm-linux-gnueabi-as
from Debian Buster) cannot compile following code from located in file
board/nokia/rx51/lowlevel_init.S:

  kernoffs:
    .word  KERNEL_OFFSET - (. - CONFIG_SYS_TEXT_BASE)

when CONFIG_SYS_TEXT_BASE is set to 0x80008000. It throws strange compile
error which is even without line number:

    AS      board/nokia/rx51/lowlevel_init.o
  {standard input}: Assembler messages:
  {standard input}: Error: attempt to get value of unresolved symbol `L0'
  make[2]: *** [scripts/Makefile.build:293: board/nokia/rx51/lowlevel_init.o] Error 1

I have no idea about this error and my experiments showed that ARM GNU
assembler is happy with negation of that number. So changing code to:

  kernoffs:
    .word  . - CONFIG_SYS_TEXT_BASE - KERNEL_OFFSET

and then replacing mathematical addition by substraction of "kernoffs"
value (so calculation of address does not change) compiles assembler file
without any error now.

There should be not any functional change.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Tom Rini <trini@konsulko.com>