msm: pcie: add support for configurable PCIe core preset

The PCIe core preset determines how aggressive PCIe PHY Gen3
equalization is. Based on the distance between SoC and PCIe
core, this setting needs to be tuned. Add support to tune
this value.

Change-Id: I1aee056cf1a81e96c2ece2c576cf27c0767f3a97
Signed-off-by: Tony Truong <truong@codeaurora.org>