ARM: imx: Kconfig: Adjust select after renamed config option

Commit 3f490a74a8a1 ("clocksource/drivers/vf-pit: Rename the VF PIT to NXP
PIT") renames the config VF_PIT_TIMER to NXP_PIT_TIMER, but it misses
adjusting a reference to VF_PIT_TIMER in arch/arm/mach-imx/Kconfig.

Adjust the config reference to the new name.

Fixes: 3f490a74a8a1 ("clocksource/drivers/vf-pit: Rename the VF PIT to NXP PIT")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@redhat.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>