ARM: imx: do not use regmap_read for ANADIG_DIGPROG

Function imx_anatop_get_digprog() that reads register ANADIG_DIGPROG is
called to identify silicon version.  Users might query silicon version
earlier than regmap subsystem is ready.  For example, imx6q clock driver
query revision in mx6q_clocks_init(), where regmap is not initialized
yet.

Change imx_anatop_get_digprog() to map anatop block and read
ANADIG_DIGPROG in the native way, so that the function can work at very
early stage.

While at it, let's move imx_print_silicon_rev() back to
imx6q_timer_init() to have the message show up a little earlier.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>