mxc: Change gpt timer code to be more generic by using V2 instead of MX3

Replace mx3_ with v2_ since the register layout is the same for all SoCs using
version 2 of the timer (mx25, mx31, mx37 and now mx51)

Signed-off-by: Amit Kucheria <amit.kucheria@canonical.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>