mtd: msm_qpic_nand: optimize nand perforamnce

The QPIC nand BAM innterface performs better when there are multiple
command elements (ce) per descriptor on the command pipe. So instead
of having one ce per descriptor, try to merge as many ce's as possible
into a single descriptor.

One thing to note is that the read/write ce's have to be split into
seperate descriptors.

Change-Id: I83c8e36e2fe8612f36270bb2250c4749ac016075
Signed-off-by: Krishna Konda <kkonda@codeaurora.org>