compat: add linux/regmap.h symlink

Add a symlink to the Linux regmap.h path so that drivers ported from
Linux don't all need additional changes to include the U-Boot specific
path.

Signed-off-by: Casey Connolly <casey.connolly@linaro.org>

diff --git a/include/linux/regmap.h b/include/linux/regmap.h
new file mode 120000 (symlink)
index 0000000..8a3780b
--- /dev/null
+++ b/include/linux/regmap.h
@@ -0,0 +1 @@
+../regmap.h
\ No newline at end of file