timers: atmel_pit: introduce CONFIG_SPL_ATMEL_PIT_TIMER

This commit allows the ATMEL_PIT_TIMER 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 0.3 KBytes.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
diff --git a/configs/sama5d3xek_nandflash_defconfig b/configs/sama5d3xek_nandflash_defconfig
index dd7713a..977eafd 100644
--- a/configs/sama5d3xek_nandflash_defconfig
+++ b/configs/sama5d3xek_nandflash_defconfig
@@ -100,6 +100,7 @@
 CONFIG_SYSRESET_AT91=y
 CONFIG_SPL_TIMER=y
 CONFIG_ATMEL_PIT_TIMER=y
+CONFIG_SPL_ATMEL_PIT_TIMER=y
 CONFIG_USB=y
 CONFIG_USB_EHCI_HCD=y
 CONFIG_USB_STORAGE=y