imx: print ARM clock for clocks command

Default print ARM clock for clocks command.
Test on i.MX6UL 14x14 evk board:
"
=> clocks
PLL_SYS         792 MHz
PLL_BUS         528 MHz
PLL_OTG         480 MHz
PLL_NET          50 MHz

ARM          396000 kHz
"

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>