ARM: mxs: detect SoC by checking CHIPID register

Both imx23 and imx28 have CHIPID register at address 0x8001c310, which
can be used to identify the SoC.  This patch changes cpu_is_xxx and
__arch_decomp_setup to use this CHIPID register than machine type to
detect the chip between imx23 and imx28, so that we do not need to
change these functions whenever a new board/machine gets added.

Suggested-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>