arm: lpc32xx: Fix timer initialization

The match controller register is not cleared during
initialization. However, some bits of this register may reset the TC if
tnMRx match it.

As we can't make any assumption about how U-Boot is launched by the first
stage bootloader (such as S1L) clearing this register ensure that the
timers work as expected.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>