timer: Add SPL_REGMAP dependency for Xilinx timer

Add SPL_REGMAP dependency when SPL is enabled. This can avoid compilation
issues if timer is selected but SPL_REGMAP not.

Reported-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/8f6c187e04cb3127bf5148ae2dbbdf55b25ea544.1655982509.git.michal.simek@amd.com