timer: cadence: Implement timer_get_boot_us

This function is required for adding bootstage support.
Also enable it directly for ZynqMP R5 configuration.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/configs/xilinx_zynqmp_r5_defconfig b/configs/xilinx_zynqmp_r5_defconfig
index 52b3f75..9e667ff 100644
--- a/configs/xilinx_zynqmp_r5_defconfig
+++ b/configs/xilinx_zynqmp_r5_defconfig
@@ -5,10 +5,12 @@
 CONFIG_DEBUG_UART_CLOCK=100000000
 CONFIG_DEFAULT_DEVICE_TREE="zynqmp-r5"
 CONFIG_DEBUG_UART=y
+CONFIG_BOOTSTAGE=y
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_SYS_PROMPT="ZynqMP r5> "
 # CONFIG_CMD_FLASH is not set
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_CMD_BOOTSTAGE=y
 CONFIG_OF_EMBED=y
 CONFIG_DEBUG_UART_ZYNQ=y
 CONFIG_ZYNQ_SERIAL=y