ARM: MXS: fix Uninitialized variable error

cppcheck reports:

[arch/arm/cpu/arm926ejs/mxs/timer.c:96]: (error) Uninitialized
variable: now

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>

arch/arm/cpu/arm926ejs/mxs/timer.c

@@ -91,6 +91,8 @@ unsigned long long get_ticks(void)
 		TIMROT_RUNNING_COUNTn_RUNNING_COUNT_OFFSET;
 #elif defined(CONFIG_MX28)
 	now = readl(&timrot_regs->hw_timrot_running_count0);
+#else
+#error "Don't know how to read timrot_regs"
 #endif
 
 	if (lastdec >= now) {