ARM: dts: msm: set 32KB buffer length for pre-allocated mhi channels

Set buffer length to 32KB for pre-allocated MHI channels to
avoid larger than order of 3 memory allocations as recommended.

CRs-Fixed: 2446873
Change-Id: Ibf81f2029c85125ae4f7165c13096bc1648213ef
Acked-by: Bhaumik Vasav Bhatt <bbhatt@qti.qualcomm.com>