goldfish_rtc: Add big-endian property

goldfish_rtc: Add big-endian property

Add a new property "big-endian" to allow configuring the RTC as either
little or big endian, the default is little endian.

Currently overriding the default to big endian is only used by the m68k
virt platform.  New platforms should prefer to use little endian and not
set this.

Cc: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Stafford Horne <shorne@gmail.com>