bitops: Fix GENMASK definition for Sandbox

In arch/sandbox/include/asm/types.h we have
Therefore for 32 bit Sandbox build BITS_PER_LONG turns out to be 32 as
CONFIG_PHYS64 is not set

This messes up the current logic of GENMASK macro due to mismatch b/w
size of unsigned long (64 bit) and that of BITS_PER_LONG.
Fix this by using CONFIG_SANDBOX_BITS_PER_LONG which is set to 64/32
based on the host machine on which its being compiled.

Without this patch:
GENMASK(14,0) => 0x7fffffffffff
After this patch:
GENMASK(14,0) => 0x7fff

Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>