The code in swapcase can be used by other sandbox drivers. Move it into a
common place to allow this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: remove inclusion of <asm/test.h> in pci_sandbox.c]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>