rtc: mxc: remove unused variable

This variable was never used. With GCC 6.2, we get the following warning:

drivers/rtc/rtc-mxc.c:44:18: warning: ‘PIE_BIT_DEF’ defined but not used [-Wunused-const-variable=]
 static const u32 PIE_BIT_DEF[MAX_PIE_NUM][2] = {

Signed-off-by: Diaz de Grenu, Jose <Jose.DiazdeGrenu@digi.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>