meta-openpower: pdbg: switch SRCREV to git hash

Latest Yocto upstream seems to prefer SRCREV to be a git-hash over a tag
name.  I cannot find any recipes in the poky tree that leave SRCREV as a
tag even when the recipe is not a `_git.bb`.  When attempting to do a
subtree update we are observing the following failure:

    ERROR: pdbg-3.4-r0 do_fetch: Bitbake Fetcher Error:
    FetchError("Recipe uses a floating tag/branch without a fixed
    SRCREV yet doesn't call bb.fetch2.get_srcrev() (use SRCPV in PV
    for OE).", None)

Switch pdbg to use the git hash of the 3.4 tag as the SRCREV instead of
the generic "v3.4" tag name.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ifdef91253c4b62003e19945a934094f8edd968d9
diff --git a/meta-openpower/recipes-bsp/pdbg/pdbg_3.4.bb b/meta-openpower/recipes-bsp/pdbg/pdbg_3.4.bb
index eefe72d..6678c0e 100644
--- a/meta-openpower/recipes-bsp/pdbg/pdbg_3.4.bb
+++ b/meta-openpower/recipes-bsp/pdbg/pdbg_3.4.bb
@@ -4,7 +4,7 @@
 LIC_FILES_CHKSUM = "file://${S}/COPYING;md5=3b83ef96387f14655fc854ddc3c6bd57"
 
 SRC_URI += "git://github.com/open-power/pdbg.git;branch=master;protocol=https"
-SRCREV = "v3.4"
+SRCREV = "becd562a5a2556c05d9a6f14df467f5d6dceba19"
 
 DEPENDS += "dtc-native"