msm: pcie: add support to change tlp read size

Change tlp read size to 4KB. Clients may wish to use a different
size depending on endpoint needs. Thus, adding support so clients
can change tlp read size.

Change-Id: I35de2b4425309b95df7d327c9641eb1555f60071
Signed-off-by: Tony Truong <truong@codeaurora.org>