Add gettime command

Gettime returns the current timer value.  If CONFIG_SYS_HZ is defined
then the timer value is also converted to seconds.

Tegra20 (SeaBoard) # gettime
Timer val: 7754
Seconds : 7
Remainder : 754
sys_hz = 1000

There has been some discussion about whether this is useful enough to
be included in U-Boot. The following boards do not have CONFIG_SYS_HZ
defined:

M52277EVB
M52277EVB_stmicro
M53017EVB
M54418TWR
M54418TWR_nand_mii
M54418TWR_nand_rmii
M54418TWR_nand_rmii_lowfreq
M54418TWR_serial_mii
M54418TWR_serial_rmii

Signed-off-by: Anton Staaf <robotboy@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>