rockchip: remove 'swiotlb' parameter from boot script

rockchip: remove 'swiotlb' parameter from boot script

We have hardware IOMMU support and this is totally unnecessary.
The given value is also unreasonable, it's too small and causes
kernel panic in some cases:

[ 5706.856473] sdhci-dwcmshc fe310000.mmc: swiotlb buffer is full (sz: 28672 bytes), total 512 (slots), used 498 (slots)
[ 5706.864451] sdhci-dwcmshc fe310000.mmc: swiotlb buffer is full (sz: 65536 bytes), total 512 (slots), used 464 (slots)

This parameter seems to be added by mistake, so remove it.

Fixes: e35c7ab51fd1 ("rockchip: merge bootscript")
Signed-off-by: Tianling Shen <cnsztl@immortalwrt.org>