msm: pcie: add debugfs cat capabilities for PCIe entries

Add cat capabilities for PCIe rc_sel, base_sel, wr_offset,
wr_val, and wr_mask debugfs entries. This provide users a
convenient way to check values written and what options are
available.

Change-Id: I3cf3eb823479c33537966a737cfc2701dd2373e5
Signed-off-by: Tony Truong <truong@codeaurora.org>