[ARM] pxa: fix NAND and MMC clock initialization for pxa3xx

After commit 8c3abc7d


("[ARM] pxa: convert to clkdev and match clocks by struct device where possible")
get_clk in pxa3xx_nand fails with -ENOENT.
Apparently, clk_get in pxamci will also fail for MCI2 on PXA310.

The 'clk_find' and therefore 'clk_get' require driver to supply
both 'dev_id' and 'con_id' if they are not NULL in the 'strcut clk_lookup',
but neither pxa3xx_nand nor pxamci supply 'con_id'.

This patch sets 'con_id' to NULL in NAND clock and MCI2 clock registration.

Signed-off-by: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Eric Miao <eric.miao@marvell.com>