powerpc/time: printk time stamp init not correct

problem:

I see sometimes on my mpc5200 based board such printk timing
information:

[    0.000000] NR_IRQS:512 nr_irqs:512 16
[    0.000000] MPC52xx PIC is up and running!
[    0.000000] clocksource: timebase mult[79364d9] shift[22] registered
[    0.000000] console [ttyPSC0] enabled
[  130.300633] pid_max: default: 32768 minimum: 301
[  130.305647] Mount-cache hash table entries: 512
[  130.315818] NET: Registered protocol family 16

reason:
if the tbu not starts from 0 when linux boots, boot_tb
maybe could not store the real 64 bit tbu value, because
boot_tp is only a 32 bit unsigned long.

solution:
change boot_tb to u64

[BenH: Made it u64 instead of unsigned long long]
Signed-off-by: Heiko Schocher <hs@denx.de>
cc: Wolfgang Denk <wd@denx.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/time.c

@@ -155,7 +155,7 @@ EXPORT_SYMBOL_GPL(rtc_lock);
 
 static u64 tb_to_ns_scale __read_mostly;
 static unsigned tb_to_ns_shift __read_mostly;
-static unsigned long boot_tb __read_mostly;
+static u64 boot_tb __read_mostly;
 
 extern struct timezone sys_tz;
 static long timezone_offset;