sh: cache use jump_to_P2() and back_to_P1() from asm/system.h

Both jump_to_P2() and back_to_P1() functions are found in asm/system.h
header file and functionally they are the same, don't redefine them.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>