i.MX51 iomux: Fixes MX51_PAD_UART2_TXD__UART2_TXD & MX51_PAD_USBH1_DATA2__UART2_TXD declarations

Fixes 2 small regressions of recent iomux changes:
 - current MX51_PAD_UART2_TXD__UART2_TXD declaration overwrites
   IOMUXC_UART2_IPP_UART_RXD_MUX_SELECT_INPUT register (0x09ec) and prevent
   UART2 Rx from working properly.
   (Tested on my custom i.MX51 board where UART2 is used as console)
 - current MX51_PAD_USBH1_DATA2__UART2_TXD declaration also has the same problem.
   (Not tested)

Signed-off-by: Julien Boibessot <julien.boibessot@armadeus.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>