Convert CONFIG_SYS_CACHE_STASHING to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_CACHE_STASHING

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/P3041DS_defconfig b/configs/P3041DS_defconfig
index f227195..0abf6e1 100644
--- a/configs/P3041DS_defconfig
+++ b/configs/P3041DS_defconfig
@@ -11,6 +11,7 @@
 CONFIG_ENABLE_36BIT_PHYS=y
 CONFIG_SYS_BOOK3E_HV=y
 CONFIG_SYS_FSL_CPC=y
+CONFIG_SYS_CACHE_STASHING=y
 CONFIG_PCIE1=y
 CONFIG_PCIE2=y
 CONFIG_PCIE3=y