powerpc: P2020RDB-PC: Separate from P1_P2_RDB_PC in Kconfig

Use TARGET_P2020RDB_PC instead of sharing with P1_P2_RDB_PC to
simplify Kconfig and config macros.

Remove macro CONFIG_P2020RDB.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/configs/P2020RDB-PC_36BIT_SPIFLASH_defconfig b/configs/P2020RDB-PC_36BIT_SPIFLASH_defconfig
index ebb89dd..7112255 100644
--- a/configs/P2020RDB-PC_36BIT_SPIFLASH_defconfig
+++ b/configs/P2020RDB-PC_36BIT_SPIFLASH_defconfig
@@ -7,13 +7,13 @@
 CONFIG_SPL_SPI_FLASH_SUPPORT=y
 CONFIG_SPL_SPI_SUPPORT=y
 CONFIG_MPC85xx=y
-CONFIG_TARGET_P1_P2_RDB_PC=y
+CONFIG_TARGET_P2020RDB=y
 CONFIG_PHYS_64BIT=y
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
-CONFIG_SYS_EXTRA_OPTIONS="P2020RDB,SPIFLASH"
+CONFIG_SYS_EXTRA_OPTIONS="SPIFLASH"
 CONFIG_BOOTDELAY=10
 CONFIG_SPL=y
 CONFIG_SPL_MPC8XXX_INIT_DDR_SUPPORT=y