imx: syscounter: make sure asm is volatile

imx: syscounter: make sure asm is volatile

Without the volatile attribute, compilers are entitled to optimize out
the same asm().  In the case of __udelay() in syscounter.c, it calls
`get_ticks()` twice, one for the starting time and the second in the
loop to check the current time.  When compilers inline `get_ticks()`
they see the same `mrrc` instructions and optimize out the second one.
This leads to infinite loop since we don't get updated value from the
system counter.

Here is a portion of the disassembly of __udelay:

  88:	428b      	cmp	r3, r1
  8a:	f8ce 20a4 	str.w	r2, [lr, #164]	; 0xa4
  8e:	bf08      	it	eq
  90:	4282      	cmpeq	r2, r0
  92:	f8ce 30a0 	str.w	r3, [lr, #160]	; 0xa0
  96:	d3f7      	bcc.n	88 <__udelay+0x88>
  98:	e8bd 8cf0 	ldmia.w	sp!, {r4, r5, r6, r7, sl, fp, pc}

Note that final jump / loop at 96 to 88, we don't have any `mrrc`.

With a volatile attribute, the above changes to this:

  8a:	ec53 2f0e 	mrrc	15, 0, r2, r3, cr14
  8e:	42ab      	cmp	r3, r5
  90:	f8c1 20a4 	str.w	r2, [r1, #164]	; 0xa4
  94:	bf08      	it	eq
  96:	42a2      	cmpeq	r2, r4
  98:	f8c1 30a0 	str.w	r3, [r1, #160]	; 0xa0
  9c:	d3f5      	bcc.n	8a <__udelay+0x8a>
  9e:	e8bd 8cf0 	ldmia.w	sp!, {r4, r5, r6, r7, sl, fp, pc}
  a2:	bf00      	nop

I'm advised[1] to put volatile on all asm(), so this commit also adds it
to the asm() in timer_init().

[1]: https://lists.denx.de/pipermail/u-boot/2018-March/322062.html

Signed-off-by: Yasushi SHOJI <yasushi.shoji@gmail.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>