Re-enable setexpr on Raspberry Pi

This patch re-enables the setexpr command, I don't really understand
why it got excluded in the first place.

setexpr can be used e.g. to implement failed-boot-counters and
failovers to rescue firmware.

Signed-off-by: Marco Schuster <m.schuster91@googlemail.com>

configs/rpi_2_defconfig

@@ -6,5 +6,4 @@ CONFIG_SYS_PROMPT="U-Boot> "
 # CONFIG_CMD_FLASH is not set
 # CONFIG_CMD_FPGA is not set
 CONFIG_CMD_GPIO=y
-# CONFIG_CMD_SETEXPR is not set
 CONFIG_PHYS_TO_BUS=y

configs/rpi_defconfig

@@ -6,5 +6,4 @@ CONFIG_SYS_PROMPT="U-Boot> "
 # CONFIG_CMD_FLASH is not set
 # CONFIG_CMD_FPGA is not set
 CONFIG_CMD_GPIO=y
-# CONFIG_CMD_SETEXPR is not set
 CONFIG_PHYS_TO_BUS=y