sandbox: define CONFIG_64BIT as appropriate ...

All 64-bit architectures are supposed to define CONFIG_64BIT to support
the relevant 64-bit MMIO accessors. The sandbox architecture is a bit
of a special case, because barebox uses the toolchain default and
doesn't force a bitness. Add 64BIT as promptless symbol, which reflects
the pointer size of the target platform.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>