MX51EVK: fix return value of get_timer_masked

get_timer_masked() should return current timestamp,
not current ticks from hardware register.

Tested on one custom board with NAND flash.
Without this patch, NAND write always TIMEOUT
because get_timer(0) return a big value.

This patch applies for u-boot-2010.06

Signed-off-by: Li Haibo <hbli@sinocastel.com>