bus: mhi: host: pci_generic: Add support for EM929x and set MRU to 32768 for better performance.

Add MHI controller config for EM929x. It uses the same configuration
as EM919x. Also set the MRU to 32768 to improve downlink throughput.

02:00.0 Unassigned class [ff00]: Qualcomm Technologies, Inc Device 0308
	Subsystem: Device 18d7:0301

Signed-off-by: Adam Xue <zxue@semtech.com>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://patch.msgid.link/20250528175943.12739-1-zxue@semtech.com