board: am57xx: Rename TARGET_BEAGLE_X15 as TARGET_AM57XX_EVM

board/am57xx supports all boards based on am57xx. Rename the taget
as TARGET_AM57XX_EVM.

Fixes: 74cc8b097d9af ("board: ti: beagle_x15: Rename to indicate support for TI am57xx evms")
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
diff --git a/board/ti/am57xx/Kconfig b/board/ti/am57xx/Kconfig
index 87654f9..cead0f4 100644
--- a/board/ti/am57xx/Kconfig
+++ b/board/ti/am57xx/Kconfig
@@ -1,4 +1,4 @@
-if TARGET_BEAGLE_X15
+if TARGET_AM57XX_EVM
 
 config SYS_BOARD
 	default "am57xx"