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>