timers: atmel_tcb: introduce CONFIG_SPL_ATMEL_TCB

This commit allows the ATMEL_TCB driver to be unselected in SPL and be
selected in u-boot proper. The SPL can use a different timer.
By having a separate Kconfig for ATMEL_TCB in SPL, the size of the SPL
decreases by 1 KByte.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
diff --git a/configs/sama5d2_xplained_spiflash_defconfig b/configs/sama5d2_xplained_spiflash_defconfig
index e1d01b4..ec2091d 100644
--- a/configs/sama5d2_xplained_spiflash_defconfig
+++ b/configs/sama5d2_xplained_spiflash_defconfig
@@ -100,6 +100,7 @@
 CONFIG_SYSRESET_AT91=y
 CONFIG_SPL_TIMER=y
 CONFIG_ATMEL_TCB_TIMER=y
+CONFIG_SPL_ATMEL_TCB_TIMER=y
 CONFIG_USB=y
 CONFIG_USB_EHCI_HCD=y
 CONFIG_USB_STORAGE=y