arm: armv7: add us timer for bootstage

solve issue when bootstage is used with armV7 generic timer
first call of timer_get_boot_us() use the function get_timer()
before timer initialization (arch.timer_rate_hz = 0)
=> div by 0

Commit-notes

When I activate bootstage on ARMV7 architecture with platform
using the generic armv7 timer defined in file
./arch/arm/cpu/armv7m/timer.c

I have a issue because gd->arch.timer_rate_hz = 0

For me the get_timer() function should not used before timer_init
(which initialize gd->arch.timer_rate_hz) at least for the ARMV7
timer.

But in the init sequence, the first bootstage fucntion is called
before timer_init and this function use the timer function.

For me it is a error in the generic init sequence :
mark_bootstage is called before timer_init.

static init_fnc_t init_sequence_f[] = {
....
    arch_cpu_init_dm,
    mark_bootstage,        /* need timer, go after init dm */
...
#if defined(CONFIG_ARM) || defined(CONFIG_MIPS) || \
        defined(CONFIG_BLACKFIN) || defined(CONFIG_NDS32) || \
        defined(CONFIG_SPARC)
    timer_init,        /* initialize timer */
#endif
.......

To solve the issue for all the paltform, we can move timer_init()
call just before mark_bootstage() in this array...

It should be ok for ARMV7 but I don't sure for other platform
impacted
- the other ARM platform or ARMV7 wich don't use generic timer
- MIPS BLACKFIN NDS32 or SPARC

and I don't sure of impact for other function called
(board_early_init_f for example....)

=> This patch solve issue only in timer armv7
   get_boot_us() can be called everytime without div by 0 issue
   (gd->arch.timer_rate_hz is not used)

END

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay73@gmail.com>

arch/arm/cpu/armv7/arch_timer.c

@@ -8,6 +8,7 @@
 #include <common.h>
 #include <asm/io.h>
 #include <div64.h>
+#include <bootstage.h>
 
 DECLARE_GLOBAL_DATA_PTR;
 
@@ -17,7 +18,6 @@ int timer_init(void)
 	gd->arch.tbu = 0;
 
 	gd->arch.timer_rate_hz = CONFIG_SYS_HZ_CLOCK / CONFIG_SYS_HZ;
-
 	return 0;
 }
 
@@ -39,6 +39,11 @@ ulong get_timer(ulong base)
 	return lldiv(get_ticks(), gd->arch.timer_rate_hz) - base;
 }
 
+ulong timer_get_boot_us(void)
+{
+	return lldiv(get_ticks(), CONFIG_SYS_HZ_CLOCK / (CONFIG_SYS_HZ * 1000));
+}
+
 void __udelay(unsigned long usec)
 {
 	unsigned long long endtime;